Whitstable is a nice little place, but very trendy, and not much to do for children. Herne bay is a bit run down and boring. But there are lots of good things to do in kent, e.g. Canterbury, Dover, lots of castles (Dover, Deal, Walmer, Reculver, Rochester, Leeds near Maidstone), Romney, Hythe and Dymchurch railway.
If you want sandy beaches, then I would recommend Broadstairs. Several nice bays, and the town is quite nice (unlike Margate, which has fairly ok sandy beaches, but is a bit of a dive!).
I think Kent is cheaper and less touristy than Dorset (although Whitstable is quite trendy, which makes me laugh, as my ancestors came from there, where they were all oyster dredgers, and it was never posh then!)