"They also feel fear, grief, sadness, hopelessness, and anger - emotions that many adults understandably still find it hard to believe, or accept, that very young children can experience.
"Babies have an emotional world, which develops in the context of their earliest relationships starting in the earliest months of life, well before they can use words to express themselves.
“Mental health begins in early childhood, which opens a window of opportunity for laying good foundations of the mind for children."
That is why PIP has joined forces with many parenting and children’s organisations, (including Public Health England and the Royal
College of Midwives), to launch the UK’s first national Infant Mental Health Awareness Week from Monday 6 to Friday 10 June.
The awareness week's focus is 'Building Babies' Minds' and the aim is to highlight the importance of laying the foundation for good mental health in the first 1001 days of life.
"As a society it is pivotal to our thinking about the experience of childhood, that we nurture our youngest citizens with a mindful environment for their physical and mental needs," said Rees.
"Infant mental health points to the fact that when this isn’t put in place for babies and toddlers there is an increased likelihood of forming early signs of mental illness."
There is a growing body of research that shows just how much of an impact early experiences can have on our mental health. According to the PIP: “from birth to age 18 months, it has been calculated that connections in the brain are created at around a rate of a million per second."
Tessa Baradon, leading child psychotherapist and parent-infant project manager at the Anna Freud Centre, told HuffPost UK that the first 1001 days of a baby's life is a pivotal time for mental health.
"What happens in earliest childhood can matter for a lifetime," she said.
"From the prenatal period (before and soon after birth ) through the first years the brain undergoes its most rapid development, and the environment in which the baby develops provides powerful experiences which form the foundation of his/her brain architecture, for better or for worse.
"Relationships, learning, behavior and health are all built on that foundation.
"Repeated, predictable positive experiences in the baby’s relationships with his parents stimulate the development of pathways in the brain that support the baby’s capacity to thrive and adapt.
"Exposure to highly stressful environments can undermine the development of the individual’s potential to learn, play, work, manage the ordinary stresses in life, and can impact their physical and mental health across the lifespan."
So what can parents do to help 'build babies' minds'?
"Recent research proves that how a parent behaves around their baby in the first 1001 days of life has a direct impact on how their baby's brain develops," said Rees.
"This is the foundation of how their mind will work as the child grows up and becomes an adult, so if a baby's brain develops healthily they are more likely to be happy and successful as older children and adults.
"When a baby has the opportunity to form a secure bond with their parent or caregiver, this can support their potential and ability to form healthy relationships throughout life.
"As a parent being sensitive and responsive to your baby's communications is one of the greatest gifts you could give to them to grow their confidence, sense of being loved and accepted in the world."
Baradon added: "Your baby is constantly communicating with you through his/her behavior. For example, when turning to you or away from you, looking into your eyes, smiling or fretting or crying, he is telling you – in his ‘language’ what he is feeling and what he needs from you at that moment.
"Try to understand what he is saying and to respond as soon as possible to that communication.
"You don’t have to get it right immediately for your baby to feel secure in the fact that you are with him emotionally and will help him when he needs you.
"One of the most important building blocks in healthy development is the ‘back and forth’/’serve and return’ experience. Little babies reach out through babbling, gestures and facial expressions, and adults usually spontaneously respond with the same kind of vocalising and expressivity.
"Providing your baby with many such experiences will strengthen your bond with him/her and will support his learning and happiness."
Baradon also explained that it's important for parents to look after their own mental wellbeing.
"Babies needs stable, safe environments, in which their parents can focus on them," she explained.
"If the parent is in emotional turmoil – e.g. marital conflict or depression – these can absorb the parent’s attention and emotional energy, and take away from their capacity to respond to their baby.
"If you are experiencing stresses – do attend to them by seeking help because you baby does notice and can be affected."
Sleep is a major concern for most parents of newborns, which can impact on both the child's and the parents' mental health.
Vicki Dawson, founder and CEO of The Children's Sleep Charity, told HuffPost UK:
"The issue of sleep is likely to preoccupy many parents in the first 1001 days, it is after all a basic need and sleep deprivation can impact negatively on parental mental health.
"It is important that parents rest when their youngsters rest and that they allow themselves time to get into a routine that suits their parenting style.
"However, it is also important to reassure parents that babies will wake during the night for a number of reasons and this is to be expected. Babies' body clocks do not develop for several months therefore the notion of ‘sleeping through’ is unrealistic for most infants."
The children's charity Barnardo’s has created a free "baby brain workout" made up of 15 activities that encourage healthy mental and emotional development.
The activities are based on ‘Five to Thrive’ principles of - talk, play, relax, cuddle, respond - which have been shown to help children develop a healthy brain.
"Babies’ minds are built through their experiences so playing with them is not just a lot of fun, it’s also really good for their brain development and wellbeing," explained Javed Khan, Barnado's chief executive.
"Happy times helps raise happy babies."
Watch the video at the top of this article to see parents trying the 'Baby Brain Workout' with their children and try out the activities below with your child.
Barnado's 'Baby Brain Workout' Activities:
TALK: Story Time
Read a book to your baby or toddler and give your child a chance to join in. If your child is older, ask them to turn the pages, predict what is happening next or get them to act out the story with their toys.
PLAY: Fun With Water
Get a bucket or bowl and put warm water in it. Sit or hold your baby safely near it and support them to play with the water. Splash each other gently or run water over their arms or toes. Respond to your baby’s facial expressions and copy them.
RELAX: Yoga Time
You don’t need to be a yoga expert to do some calming stretches. Stretch your
arms up and down and use a calm voice to tell your child to copy your movements. Babies can take part too, gently help them to get their arms up for a good stretch.
CUDDLE: Tickle, Tickle
Tickle their toes, palms or any other areas they like and get them to tickle you
too. You can also try out ‘high fives’, ‘pat-a-cake’, ‘this little piggy’ or any other
childhood game that includes contact.
RESPOND: Copy Them
Respond to your child by copying their facial expressions and give them time to respond. Playing these games is great for your child and can be amusing and amazing for you too.
More activities can be found on the Barnardo's website.
The organisations behind Infant Mental Health Awareness Week have come together to call for more investment in looking after infants' mental health in the first 1001 days.
"The '1001 Critical Days Manifesto' is calling for increased support for vulnerable families where mental health, domestic violence or substance misuse may be present around children’s experience of early childhood development," explained Rees.
"To ensure that every baby has the best possible start in life and opportunity to develop good mental health in infancy alongside support needed for parental wellbeing.