If you have young(ish) children, and don’t mind driving abroad, then I’d highly recommend Duinrell, Wassenaar in Holland.
Plenty to do, but relaxing as well. Kids love it. Parents/Grandparents can do as much or as little as they like. Accommodation varies from Tents to Static Caravans to Chalets, and can be had at quite reasonable prices. We’ve been twice, and both times sailed from Harwich to the Hook of Holland, which leaves only a one hour drive when you dock. Alternatively, you can sail from down south into France.
Added bonus??? The Dutch Pancake Houses!!!! Yummmmmmmmmyyyyyyyyyyyy!!!!!
:D