DS has juice sometimes, he has it at every party at preschool and at birthday, and we offer him a juice as snack when we are out and about, sometimes. We introduced juice last year, really, before it was only water, and now if he is thirsty he asks for water. He never stopped drinking water for juice. So, if you think they are not getting enough liquid and that a bit of juice could help this, I would not say no, maybe I would limit it on a special time of the day (like with dinner or with morning/afternoon snack). Could you also offer homemade orange juice?