Tenby in Pembrokeshire. Fantastic beaches. Oakwood theme park. Dolphin watching boat trips, trips to Caldey island where the monks are, trips to Skomer island to see the puffins. Go to St Davids to see the cathedral. Folly Farm just outside of Tenby which is meant to be one of the UK's best farm places. There is a dinosaur world place which your ds would love. Pony trekking.
North Norfolk - nice beaches again, crabbing off the quayside at Wells, lots of national trust type places, Sandringham, hire at boat on the broads for the afternoon, Bewilderwood which is the most famtastic treetop adventure playground ever, shire horse centre at Runton, Denver windmill, boat trips to see the seals, loads of little steam railways, alpaca trekking at Wells next the sea, Amazonia zoo place. You could stay somewhere near Cromer.
We have been here in the winter and are going in August. Its a macdonald resorts place, really lovely. Close to villages / beach etc in gorgeous Snowdonia. We got a deal through laterooms for £100 a night for a bungalow or villa that sleeps up to 6. Previously we have stayed in mansion house apartment (lovely) that sleeps up to 4. Its got outdoor pool, and 9 hole pitch and putt, and indoor pool and kids activities. We love it there. Its normally over 1k for a week in school holidays, but we were made up to find it so cheap on laterooms. Can get mansion house apartments for 85 a night some dates.
Good luck hope you choose something fab anyway and have a great hol