Children aged two or younger are being allowed to use the internet, while many youngsters are spending much longer online than their parents realise, a new poll suggests.
The survey reveals that almost four in five children (79.8%) have seen images or information about eating disorders and self harm on the internet, and over two in five (42.1%) admit that they have seen online porn.
Others have seen content on suicide, child abuse, animal cruelty, gambling, terrorism or cults.
In many cases, children accidentally accessed this inappropriate content, but almost one in 10 (9.4%) revealed they found it on purpose because they were looking for it, while a further 18.1% found it by following links they were curious about.
The Netmums survey questioned around 1,100 parents and about 825 children aged seven to 16 for their views on the internet.
The findings show that three quarters (72.8%) of parents believe that their child spends under an hour a day online.
But an analysis of the children's poll suggested that on average, youngsters are spending two hours each day on the internet, Netmums claimed.
Around one in eight (12.8%) of the parents surveyed said that their son or daughter was two years old or younger when they were first allowed to go online, while 16.7% said that they let them use the internet at age three.
The survey also found that almost three in 10 parents (29%) let their youngster use the internet without any restrictions or supervision.
More than three quarters (77.1%) of the children questioned said that they had not acted on information they had found on an inappropriate site.
But 17.3% said it made them think about acting on it, while 5.4% said they tried to restrict their eating, 3.4% self-harmed and 2.1% considered suicide.
The poll asked children for their experiences online and found that almost one in four (24.8%) have been bullied, 43.9% have pretended to be older to get an account on a particular site, 12.4% have received a sex text from other people, while 6% said they have sent sex texts, and 7.1% have met up with a stranger they met over the internet.
Netmums co-founder Siobhan Freegard said: "No past generation has ever had access to so much information so fast - and not all of it desirable - along with the use of dozens of different electronic gadgets.
"Some scientists have predicted large amounts of internet exposure may alter the way kids think and in many ways we are in the middle of a giant experiment with our children as the subject. We are raising a generation of technical geniuses - but at what cost?"
A Department for Education spokesman said: "The government-led UK Council of Child Internet Safety has helped to ensure the four main ISPs now offer all customers free and easy-to-use parental control tools to help protect children from accessing harmful content.
"In addition, under our new curriculum children from the age of five will soon be taught at school how to stay safe online and communicate safely and respectfully."