I wouldn't give apple juice, or any juice for that matter, because I would be worried about dental issues, like teeth decay. There is a lot of sugar in fruit juice, and that sugar is what does the damage. Tooth decay can lead to all sorts of issues, such as an infection in the gums which could end up requiring surgery or worse, death.
In my opinion, plain water is the best and healthiest option. (And that goes for humans too!)