Is is possible for you to force your little girl (by showing cartoons or characters which she like) of eating fruits with lots of water content like melons? She definitely has some fear (due to previous UTI - fear of pain while passing urine, in previous instances) and that forces her to hold her bladder as long as possible. Do speak to her that nothing is wrong with her and that she is fine (as with frequent visits to doctors and tests give an impression that something is wrong with her). My daughter is 3.4 yrs old and even when bruised to a great extent, when told that its a minor thing and brushed aside, she gets off it. So please do try.