The disturbing trend has been highlighted in NHS figures which found rising numbers of youngsters suffering in homes where there is unemployment, financial problems or drug abuse.
One trust which covers Southampton and Portsmouth has had 378 children below 16 undergoing therapy - with one patient aged two.
Nationally, one in 10 children aged between five and 16 has a clinically diagnosable mental health problem.
NHS Solent clinical psychologist Barbara Inkson said: "Levels of emotional disorders are rising."
Mental health workers in Sussex are working with about 330 under-11s suffering from anxiety and depression.
Sarah Brennan, chief of charity Young Minds, said:
It is vital that we invest in children and young people's mental health to prevent them suffering entrenched problems as adults.
The figures also found that prescriptions for Fluoxetine, more commonly known as Prozac, have risen 26 per cent in Oxfordshire and 13 per cent in Berkshire from April to September last year.
Young Minds, said it wants more treatments based in school and the community rather than in mental health units, which it says can be intimidating for children.
Sarah Brennan added: "Intervening early when a child or young person starts struggling to cope is proven to reduce the likelihood of that young person developing much more severe and entrenched mental health problems."