Some 28% claimed to have witnessed kids facing negative behaviour at a sports club or activity.
As a result, 68% of these parents said they or their youngsters had felt uncomfortable because of it but almost one in five parents said no action had been taken to stop it.
The NSPCC, which did the study, wants parents to recognise negative behaviour can impact their young ones and their pleasure in the sport.
Sir Peter Wanless, boss of the leading children’s charity, said: “Being your child’s biggest supporter can help instil in them a lifelong love of sport and help them build friendships and confidence.
“It can also help you learn about what makes a club safe and how to spot signs something may be wrong.”
The NSPCC’s campaign, Keeping Your Child Safe In Sport Week, aims to highlight how important parents are in their youngster’s sporting life.
Sir Peter added: “You can make a difference by being involved and supporting a child in a positive way.”
The charity’s campaign has been supported by sports clubs and stars. Skateboarder Sky Brown, 15, the youngest British Olympian, said: “All parents should feel they are equipped with the information that they need so that their kids can be with a safe sports club where their kids are having fun.
“Having my family with me is really important to me on my journey. My dad and my brother are always by my side and mum makes me feel at home when we are on the road and cooks the best food.”
Alan Shearer, 53, former England footballer, said: “It’s really important we support children in sport.
“It can be an emotional experience for young people. It’s vital adults are around every step of the way.”
Football clubs Liverpool, Everton, Leeds United, Sunderland, Birmingham City and Tranmere Rovers plus the Lawn Tennis Association and Volleyball England also back the campaign.
The NSPCC’s Child Protection in Sport Unit wants to bring together parents and sports clubs to help give knowledge about the role they play.
Michelle North, head of CPSU, said: “We know that inappropriate behaviour has a negative impact on young people and affects their enjoyment of sport, which in positive and safe environments can bring a lot of fulfilment.
“It is important sports organisations and activity providers not only work with their staff and volunteers but with parents.”