You have the beach front fairground. It is small but my DS loves it. He is six. There is an aquarium, sea life centre. The smugglers caves are pretty cool. The castle and the information about in the battle of Hastings is good if you enjoy your history. The castle is a ruin by the way.
A few miles away is Battle where the actual battle of Hastings took place, alongside Battle Abbey. There is an exhibition there and the battle ground. The ground is literally a field but there's a good audio guide which, if you have the imagination, can take you right back to the battle times. Across the road from Battle Abbey is a Yesterday's World (I think that's the name). Lots of stuff going back into the past. DS loves it there. He likes all the toys and the little play area outside for the kids. Nice tea room there and plenty of cafes outside the attraction in the high street. Parking is a nightmare sometimes but you can, or could, park free at the Abbey if you are a member of English heritage.
Slightly further afield is Rye, lovely hill top town and Rye harbour about a mile from Rye town has a nice marsh walk where you can take bikes or kids scooters.
Half an hour or so from Hastings is Bodiam castle. Though that doesn't have floors it isn't as ruined as Hastings. There are river boat rides there and the kids can play around the fields round the castlr with the sheep and ducks. They wander free - the sheep and ducks not the kids ha ha! You could get a return steam train trip from there to Tenterden on the steam railway. The station is about three or four minutes walk from the castle.
Half an hour along the coast from Hastings is Camber Sands. Very sandy beaches and then tide goes right out so rock pools etc are formed. Hastings has a pebble beach so if you want sand head for Camber
Further afield, about an hours drive, is Groombridge Place. Probably one of our favourite places. You can have a culpa with peacocks walking by, have a go at giant chess, walk around the gardens, learn about the association with Arthur Conan Doyle and then take the kids into the woods to find the raffia dragon, the giant swings, huge rope walkways and bridges, gipsy caravans, riverboat rides, play areas etc. Then you can walk back to your car looking out for the resident deer herd. TaKe very comfy shoes!