Ah, thanks for the lower leas link martis, i think we are to go there soon!! 
Broadstairs, fab beach, just a lovely lovely town. If the weather is pants, head up to rochester, we went there last week, there is a castle, and a museum which is free and really great (kids will like this, they have a "boat" for them to walk around). Good shops, riverside walk.
Whitstable - just lovely, some great crabbing to be had off the jetty, stoney beach (much better than sand if you ask me)! The harbour has a market at the weekend, a bit expenevie and pretentious, but nice to stroll around.
Herne bay is good if you like amusements and cheap and cheerful type places.
Margate - i love margate, yes, its a bit run down but the old town is good, it has a great beach, give it a chance.
Howletts and Port lympe are EXPENSIVE, as they charge £20 each because its a yearly ticket, so only really any good if you want to visit more than once. They are good zoos and their apes and gorrilas are enchanting.
Canterbury itself is a great CITY and definately worth a visit, not that much for the kids really, there is a rupert bear museum and canterbury tales museum (my DD too scared to go in though).