My dd drinks a lot too. So much so that during the summer, we were having leaks after half an hour during the day. I went to my health visitor and he did a test for diabetes (urine sample) but everything was fine. He told me to limit her fluid intake as he thought it was attention seeking behaviour. I tried it for one day, but had an unhappy, screaming and very constipated toddler. I guess some children just need to drink more. I'd get it checked out, but don't worry too much. The hv did say when he tested her that I would probably have noticed other symptoms too.