If your toddler had UTIs tell me about what happened and when they stopped? What did you do?

We are potty training and I think my 3 year old is getting her 3rd UTI!! :(:: The first time she held it for 6 hours... I tell her not to hold it in, make her drink lots of water, get her to sit on the toilet frequently, explained why, etc... But she still holds her pee. She seems to hold it in more at daycare. Even when the teachers take her quietly by herself, she refuses to release. She has no problem with sitting. I'm so scared she will damage her organs at such a young age. Tell me about your experience and what you felt helped stop them?