There is a small waterpark in Calis which on first appearance you might not think is worth visiting. When we visited in June our apartments were fairly quiet and the only children that were there werent around much. We have two children aged 7 and 5 who love to meet other kids on holiday so we went to the waterpark. Entrance was TL 68 for the family for the day and you can leave the park and come back if you want to. Food is good and reasonably priced and there is also a small shop selling inflatables etc.
There is entertainment for the children during the day such as games, dancing to Agadoo etc. There is a pool for small children as my youngest doesnt like the big slides.
There are some photos here (not mine)
http://community.webshots.com/album/110610149HebuEO