Only recently (like the last fortnight) has DD1 been dry overnight. She's 6.5.
There is a hormone that the kidneys release that tells their body not to make wee overnight. The release of that hormone is developmental. Not something they can control. Withholding drinks or taking them to the loo overnight has no effect on the hormone.
We had some success with an alarm mat (so nothing tied to her), but essentially I think it was something she just grew into.

Give your local community health centre a call if you're worried. Most of them have a continence nurse that you can chat to.