The Island is not that big, if you have a car, you'd never be more than 10 minutes from the sea. So whilst Ventnor would be a good option to stay, any where on the Island would be as good.
If you do stay at Ventnor, it is a steep walk down to the beach from the town.
If you don't have a car, stay at somewhere on a bus route, buy a family rover ticket- there's some fantastic scenery to be seen from the upper decks.
depends on how old your children are. maybe sandown which has a two mile sandy beach, a pier with amusements,ten pin bowling,indoor crazy golf,dodgems,cafe and bar. the town also has a go kart track,crazy golf and a pitch and putt and tiger zoo and dinosaur museum.
the island is small and easy to get around either by car or bus,so you could visit other days out places like blackgang chine,alum bay,amazon world,seaview sealife centre,robin hill ect and if your interested in history queen victoria's former residence osbourne house.