Holding in urine can increase the risk of a urinary-tract infection -- when the pee just sits there, there's a higher chance that bacteria could get in and start multiplying. It's a pretty small risk, though. Actively holding it in isn't a good idea, but some kids go a long time between bathroom trips simply because they have a large bladder or they don't drink very much (that's the case with my 8-year-old daughter). I would certainly encourage your daughter to down plenty of fluids and to visit the bathroom regularly, even if it's just to try. And instead of nagging her to do it, be lavish with your praise whenever she does make an attempt -- you'll get better results. Otherwise, I wouldn't stress about it. The ability to hold pee for hours can actually come in very handy, like on long car trips or when you're in public places with icky bathrooms!
Claire McCarthy, M.D., is an assistant professor of pediatrics at Harvard Medical School in Boston. Send her your questions at Parenting.com/momsquad.