Wells beach, Holkham beach - the nest beaches in the world, but they are simply miles of sand and dunes, and 'Gruffalo woods' - duney pinewoods with great climbing logs etc.
From the Wells beach car park, you can go on the boating lake or trampolines by the caravan site. Ot hire a beach hut for the day from Pinewoods caravan Site (google it and book in advance) and pretend you are in a colour supplement article - they are amongst the most photographed huts in the U.K!
Shire Horse Centre, Sheringham, ROOTS museum of old village and farming life, Thursford Steam Engine Museum with steam organs and a couple of roundabouts.
The thing about N Norfolk is that it is about the great outdoors, rather than 'attractions'.
A seal trip to Blakeney point would be great (book from Blakeney Quay or the Pub in Morston, boats leave from Morston) but if you want to enjoy Blakeney point at low tide, you will have to walk back, 3 miles along shingle to Cley, or over mudflats to Blakeney - so a trip for the hardy. (actually, I feel I should say that no-one without local knowledge should attempt the route over the mudflats in case future MN-ers end up stranded on an incoming tide)
The Henry VIII pub at Letheringsett has won awards for it's child-friendliness- has a play castle etc.