There are some great suggestions on here- have been to many.
I would also suggest River Dart with the age of your DS it would be perfect. It's quite a big site, but we stayed twice and never had a problem with noise, although I would choose a pitch wisely here. I's expensive in comparison with other sites and quickly booked up, I am sure I booked November for one of the last electric pitches in May half term. There's a lot to see around the area.
We have also stayed at Cotswold View in Charlbury- farm, playground and skittles on site, cafe and a good base for attractions such as Oxford, Blenheim palace, Warwick castle etc.
Duinrell in The Netherlands- free theme park on site (and water park, if you book early you get free access to that as well). Well placed for day trips out- again expensive but very much worth it. Can be a bit noisy.
The Quiet site is good, a bit of a dog leg to get to places to visit. Bathrooms were good, they even had baths!
The Hideaway has fantastic bathrooms and a good vibe, it's near York and Pickering, Scarborough, Knaresborough, Harrogate etc.
We stayed at Upwood Park near Haworth this year, that has a playground and football field, and pub on site. Bathrooms were excellent. It has quite a few seasonal caravans. Quite a bit to do in the area.
FForest fields is lovely, but there is no playground (which I prefer), but if your DS likes riding a bike, swimming or canoeing in a lake, and exploring then it is ace- if he makes friends easily he should be able to there. There's not as much to do in the immediate area, but if you're happy to travel about an hour there are more attractions.