coolcat. another good place near you (?) is Framlingham castle. i haven't been for a few years but it was quite good for a castle. its another property run by the english heritage.
if you fancy a weekend away two really good open air museums are "blist hill" ( near Ironbridge) and "the black country living museum" in Dudley. i could not fault either of these places. your children may be too young at present to appreciate the places but we as adults loved them. at the black country museum we went on a special weekend. there are lots of houses here that have actually been knocked down then rebuilt here.
you have houses from the 19th century up to the 2nd world war. each house is kitted out as a house of that time. when we went people were living in them for the weekend. there was a victorian school where we went and actually had a lesson. brilliant is all i can say. shame its not nearer. medot.x