I hate to be a downer on this, but I feel the true need to be a bit serious about this. If you have a child who has ever had even ONE UTI, then please do not ever let that kiddo in a hot tub.
My 4 yo princess is about to lose her left kidney later next month (we are going to WDW first so that we are all in a happy place before entering a difficult time).
Now, I am not saying that her going in a hot tub caused her to suffer kidney damage...however, due to her condition I have done a ton of research and am very well schooled in the the perils of uti's, kidneys, hot tubs, and all of these issues. So please, the hot tubs say "not for kids under 17" for a lot of reasons. Protecting your kids' urinary system is one of them.
Peace.