I'm Danish! Prague is lovely, but I can offer lots of advice about Copenhagen, my hometown. Brits tend to like it, and they all speak English. It's not the cheapest place, although things like the zoo and amusement parks tend to be cheaper (around half price compared to England).
AirBB works very well in Copenhagen, although don't expect it to be cheap.
The original Legoland is in Denmark, it's not close to Copenhagen, but it can be done, either by bus/train or car hire.
The beaches are lovely, and because Copenhagen is so small, there is always one near.
Go on the canal boat rides, they're great. There is a new and very well equipped playground with water features in Fælledparken, fantastic place. In Kongens Have there is a really good puppe theater daily (apart from Mondays I think), for free.
Copenhagen is generally very family friendly and easy.
Please feel free to ask me anything!