Yay Norfolk!! When all 3 of our boys were under 5 we had 2 holidays in North Norfolk and they were both great.
The first time we stayed at Pinewoods holiday park in Well Next The Sea. We got the most "Luxurious" mobile home we could get and it was spotless, quiet and perfect. There is a mini steam train that runs you from the town to the site whichj the boys loved. Down the road there is also another narrow gauge steam train from Wells to Little Walsingham which is a nice day trip too. (There is quiate a posh farm shop there selling local produce).
Wells itself has a great beach although the tide comes in mega fast so you have to sit up near the beach huts. You can hire a pretty beach hut from reception at Pinewoods to save you lugging your stuff there every day.
The second holiday we stayed in Cog Cottage in Great Walsingham which was absolutelyn gorgeous and perfect for our needs. The boys had bikes and rode through the forded stream a lot and had great fun.
I also know a family who have stayed at Bosuns Rest and loved it. (They have babysitting on offer too)
If you tire of the beach, the town of Holt has loads of nice shops and cafes and a paint your own pottery place. The villages of Cley and Blakeney are lovely and good for seal watching trips in boats!
In Wells itself its lovely to go to the Crown Inn which is on a safe green where your kids can play whilst you eat on tables outside.
There are 2 attractions a little further afeild which are great on an overcast day.
Dinosaur Adventure, which is great, non-tacky, and fun.
And BeWILDerwood which is fantastic fun if your kids are old enough.
I can't rate these holidays we had highly enough and think, if you have good weather you really can't do better.