Hi! Our kids may have the same issue. My daughter, now 8, had severe urinary reflux. She was also rushed out of diapers and on antibiotics daily for years. Her left kidney is very damaged and only functions at 26%. We took several Disney trips during her hard times and they all went off without a hitch! My husband would hold the place in line and I would take her to the bathroom as necessary. Yes, some people give you snarky looks. But your child deserves to enjoy the magic just as much as everyone else. I've had to explain the condition to CM's and they have always been fine with me leaving the line. Sometimes I would tell the person behind us that she had a kidney condition, just out of courtesy so they would understand. I would always try to stop at the bathroom A LOT! At least once an hour, and that helps a lot. I know it's inconvenient, but if you just stop at the bathroom whenever you go by I think that you'll be surprised to see that you won't have to leave the line very often. As other have said, do your best to plan well and ride the attractions that carry long lines early in the morning or late in the evening.
And just to let you know, my daughter reimplantation surgery to correct her reflux when she was 4 and hasn't had any problems since then. And we haven't had any problems with the low functioning kidney.