I would say that if you are getting enough wet nappies in a day then you don't need to worry about dehydration. If there aren't many wet nappies try some water. I introduced small amounts of water from about 6 months. Now at 18 months DS drinks only cow's milk (3 times a day) and water the rest of the day (as advised by paediatrician, MCHN and dietician). I had heard somewhere that I should introduce juice at 18 months so asked my friend who is a dietician and she said definitely NO. She said juice is too concentrated and too high in sugar (even the ones with no added sugar). She also said that it's risky to introduce sweet drinks as he might then want them instead of water. She said a piece of fruit with a drink of water is the way to go. HTH.

Melanie