There is really no need to. Dragons don't drink much, if at all. As long as they are eating properly, they get all the moisture they need from their veggies and the insects they eat. If they need extra moisture, they will absorb it through their vent when you
bathe them.
Fruit juice is filled withe a lot of sugar, and even natural sugar is something to avoid, which is why fruit is an occasional item in dragon diets. IT doesn't seem like it would be beneficial to give them watered down juice.