Clacton & Cromer airfields are within easy reach of the coast, walking or otherwise. There's good walking/biking at the former, suitable for youngsters and more than the stereotypical bucket and spade attractions in the area.
Cromer has its crab and lobster reputation and the children might like looking round the lifeboat station at the end of the rather short pier.
I don't know what length of runway you're happy with and both are grass, as is Maypole which is a short step from a good pub, dog friendly, & lunch.
Others will be along with more ideas soon...