This article by Ben and Dinah Hatch and their children covers what they consider to be the top ten visitor attractions for people with children. None are free, which is a bit (a lot?) of a drawback. However they obviously come highly recommended, so it might be for you if you have kids of your own as well as your mindees or have an agreement with your parents that they'll stand any reasonable extra costs. In the main article there are links to the top ten attractions on a regional basis.