Karina Roberts, aged nine, can no longer play in the park near her home in Prestatyn, Clwyd, after being repeatedly bullied by older children.
Such has been the concern about the intimidation that mothers using parks in the town have drawn up a petition calling on Rhuddlan Borough council to provide better supervision.
Mothers have complained that as well as physical violence and name-calling the young bullies operate an extortion racket, taking money off children to allow them to use what should be free facilities.
Sheena Roberts, Karina's mother, said: "We thought it was important to highlight what was going on. Young children's lives are being made a misery when they should be enjoying the summer holidays. Karina is too frightened to go out to play now."
Delwyn Tatum, head of the Countering Bullying Unit at Cardiff Institute of Higher Education, and an authority on the subject, said the Prestatyn incident was not an isolated one.
"Where do bullies go in the holidays? They don't disappear. They just practise their bad behaviour wherever young people gather," he said. "In some ways the potential in school holidays is worse because there is less supervision and it brings together children of primary and secondary school ages."
He said that while local authorities had rightly become concerned about the safety aspects of children's play areas, bullying was still to be tackled. He felt there should be better supervision and anti-bullying material should also be on hand at areas like parks where children gather.
Mr Tatum said: "At the park in Prestatyn there is no one with any responsibility for what is going on.
"Parents who are supervising their own children could perhaps watch out for any bullying incidents. The whole issue of bullying in the holidays needs investigating and it is a subject the unit will be tackling."
Mrs Roberts fears the problem will get worse and the mothers have formed a Families Against Bullying group to make teachers aware of the issue.