The 15-year-old boy allegedly pushed the girl inside a toilet cubicle and locked the door, stopping her from leaving, at McDonald Park on Station Road about 5.30pm Saturday.

Police have been told the girl's mother was trying to force the door ajar when the boy opened it and ran away.

Officers from Lake Illawarra Police District attended and arrested the teen at a nearby home.

He was taken to Lake Illawarra Police Station, where he was charged with take person intend commit serious indictable offence, and common assault.

He has since been granted conditional bail to appear at a children's court on March 10.

Word of the alleged incident spread quickly on Sunday after the girl's mother posted a warning online about "a 15-year-old dirty little b--tard of a boy [who] held my daughter against her will in the toilet block".

"Luckily my instinct told me something wasn't right [and] me and my husband chased the boy ..." wrote the mother, adding her daughter had walked away scared but unharmed.

"But imagine what could of happened [if] we weren't there to watch," she wrote. "It only took a sec for this to happen. Please watch your kids. Be mindful that not everyone at the park has good intentions and not all predators are aged over 18."

Parents visiting the park with their children on Monday told the Mercury they were alarmed by the alleged incident.

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Dapto mother Christine Cygan said she wouldn't let her daughter visit the toilets alone any more.

"I was quite shocked, quite disgusted and a little bit scared because we come here quite regularly," she said.

"It makes it even scarier [that the girl's parents were nearby]. The (accused) was [allegedly] just so brazen about it, so blatant. You just wouldn't expect it to happen.

"It's not unusual for me to just watch my daughter wander off to the toilet by herself and back. I'm hesitant now to let her go to the toilets alone again.

"I'll be a lot more vigilant now with where my kids are going and what they're doing.

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Another parent told the Mercury she felt the park was generally unsuitable for younger children in the late afternoons, when large numbers of teenagers would visit.

Local dad Cameron, parent to a three and six-year-old, said he already accompanied his children to the toilets and would continue doing so.

"It's extremely scary," he said. "You've got to watch your kids like a hawk."

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