Sounds to me like one of the girls I babysat years back. She had just recently been potty trained when I found out she can't have too much fun outside.
The first couple times, she would come in wet, so I would ask after a while if she had to go. Of course the answer was no, then suddenly she was squatting down holding it. I asked again if she had to go and she still said no. Then she didn't have to any more.
My solution - Have an accident, it's time to go in for the day while the others stay out. For a little while, she would try to wait as long as she could and not make it, but eventually she learned to just give up and go in when she had to.
It's so hard for kids to stop having fun just to go to the bathroom. They all figure it out and realise there is a limit to how long they can hold it.