My all time favourite place in terms of family friendliness, is the White House Hotel on Herm Island. Herm is one of the smallest of the Channel Islands, and going there is like stepping back in time. No cars/traffic/noises, just beautiful coastal paths and beaches. The small hotel is amazingly family friendly, with children staying absolutely free most of the year on a half board basis. They have a special children's high tea so that the adults can eat in the restaurant later in the evening. They have baby listening in all the rooms, and can also arrange babysitters.
There is a very well priced children's menu for lunches, a well stocked games room with lego, jigsaws and puzzles as well as board games, and an outdoor pool which is safe and warm and has the most amazing views from it. The staff adore children, and possibly the best time to visit is Easter Sunday, when the Easter bunny himself visits, and there is a huge (free) Easter egg hunt throughout the hotel gardens. The staff love children and are so welcoming. It really is a lovely place to stay with children.
Without going so far afield, I think that Morwellham Quay is one of the loveliest places to visit with children. I took my DCs there aged 6 and 8, and we all enjoyed ourselves so much. Learning is very much disguised, so as the children have fun exploring the site (and the fun of the narrow gauge railway), they are also being exposed to a huge amount of history. There is the opportunity to dress up which is always a hit. The cafe serves delicious, healthy, home cooked food and is clean and comfortable.
If I wasn't due DC3 any day, we would be heading back there asap!