I'd go Brighton. Folkestone is not that great anymore and steadily goes down hill all the time. With Brighton as well you've got Worthing and Shoreham near by which are both lovely areas to. Theres a great park in Lancing (i think) which is close to brighton that has a train that goes all round the park. Fantastic in the summer!
Opinions please. We now live in scandinavia, and are visiting the Brighton area in early July, with two DCs 4 and 6 yrs. Although I used to live in London I don't know the south coast so well. Could we make a summer holiday of it? Thinking maybe a caravan, beaches, just nice and relaxing, pottering, going to swimming pools etc. Would you recommend it, where should we go?