Fort Fun is one and Treasure Island is the other. Also the Sovereign Leisure Centre just down the road.
I haven't been to any of them for several years so can't advise on which one is best, but driving past, they all look pretty good. Plus you can go and have a picnic on the beach which is right next to them!
I've got good friends in Brighton, so I know it quite well
I'd go to the Pier (some people hate it but I love it) Walk along the Prom Have lunch in one of the many places to eat or drink either on the seafront or in the Laines Get the electric railway train that runs along the beach Stop at the playground which has a nice beach-style bar next to it Go on the beach if the weather is nice Do some shopping if the kids fell asleep (great shops in Brighton!)
There are also some really lovely parks in Brighton
Obviously that's a lot for a day trip but there's lots to choose from
Brighton has two parks on the front one near the old peir which has a coffee shop and snack bar and wine bar and ice cream stand; take the kids swim wear a paddling pool and sandpit all in one space and the other one is down the other end on the way to the marina where the train ride goes to that park which is called peter pan park and it has sprinklers which the kids love and a coffee and ice cream hut really nice and friendly good day out.
Eastbourne has a pier and nice parks too! But I'll admit, the shops are better in Brighton!! Central Brighton car parking is extortionate too (unless some Brightonian knows a secret cheap central place to park, in which case I'll be forever grateful!!)
Cattleprod, you can park for free at the multi-storey car park at the Marina. It may now have a limit of up to 4 hours or something, double check that. But from there you can get the Volks Railway along the beachfront to the Palace Pier. You can stop half way at the Peter Pan playground and Yellowwave Beach Volley place. There are water sprinklers, a cafe, toilets at the playground and next door at Yellowwave, there's another cafe, beach volley, and a small climbing wall for children. You can also get a nice glass of wine at yellowwave
You're welcome. You could also consider coming back to the Marina for lunch. There is a whole selection of restaurants up on the boardwalk overlooking the boats, such as Pizza Express, Zizzi's, Frankie & Bennies, Cafe Rouge etc.