Just back from the Ickworth hotel in Suffolk, with kids similar ages to yours, and loved it!
Two hours crèche per day included, the kids enjoyed it, it was really well staffed with about 2-3 lovely ladies looking after max of about 5 kids.
Indoor pool lovely and quiet and warm. No slides etc but there are floats and toys for kids.
Playground, bikeshed full of bikes and scooters to borrow. Gorgeous grounds.
Breakfast and lunch are good, nothing amazing for a hotel, but the adult evening meals are fab.
We were lucky that our video monitor worked from the bedroom to the restaurant on the Wifi, though you can also use the audio monitors that the hotel provide.
There is a national trust property adjacent with great shop, cafe, tours.
It was really quiet, you got to know the faces of the other kids and parents.
There aren’t lots of staff around, you have to go seeking people if, e.g. you want to order drinks or food in the lounging areas, however once you’ve asked for something it comes quick, and the staff are all very nice.