I live on the coast near Dover, so may be able to help you here.
Dover castle is great, there's lots to do & see there, but be prepared for lots of walking!
Deal castle is a bit boring IMO, but Walmer castle has lots to see, and it has great gardens. My boys love running around in the grounds of Walmer castle!
If you want a sandy beach, you are best of heading to the Thanet area, where there are great sandy beaches. Deal, Dover & Folkestone beaches are pebbly.
There are soft play areas in Whitfield (nr Dover)and there are a couple of good ones in Folkestone. (Huff & Puff in Folkestone is a good one) There is also a great outdoor adveture playground in Folkestone, which has loads to keep the children entertained.
Kearsney abbey is a lovely park in the Dover area. You could easily spend a whole day there when the weather is good!
Deal has a swimming pool called Tides, which has slides & a wave machine. It also has a gym, which offers special baby gym sessions for the under 5s on Tuesday, Wednesday & Friday mornings.