I went through this with my daughter until she was 7. After testing her for everything, including diabetes, I finally went to see a Naturepath. What she told me worked IMMEDIATELY to stop the problem...
The Problem: The bladder releases when blood sugar drops so keeping her blood sugar levels up is the key to her sleeping through the night.
The Solution: Give her a substantial high fat/low sugar snack before bed. This will regulate her blood sugar levels throughout the night.
Water should be given liberally before bed because dehydration releases the bladder as well (the brain thinks it is starving and blood sugar drops when dehydration sets in).
Bedtime snacks high in protein include: Cheese with bread or crackers (or just cheese), peanut butter or nut butter with bread or crackers, bean and cheese burrito, fish ~ tuna on toast... what ever you child will eat that is high in fat/protein and low in sugar (make sure your peanut butter is not sweetened).
Water, water, water ~ allow her to drink until she isn't thirsty anymore.
You won't believe how well this works. Start today and she will be all over the bed-wetting literally over night.