Three-month-old with renal agenesis

Patient: I have a 3-month-old daughter with a solitary kidney. She is currently taking the generic for Bactrim 3 ML a day for 7 days and after that she will take 1.5 ML until her next doctor’s visit on Dec. 2nd. Is this medicine generally safe for someone of her age with only one kidney? Also, on the pamphlet and on the bottle, it says to drink plenty of water, but she is only 3 months, so obviously that’s not an option. What should I do? Thank you in advance for your time. – Jessica