Bodiam Castle (National Trust) is a 'real' castle with turrets and a moat. Hastings has two cliffs with railways. One cliff has a castle, the other a country park. Battle Abbey (English Heritage but you can walk around the site on footpaths) and the history of the Norman Conquest (but would probably not appeal to a 3.5 yo). Beaches tend to be shingle. I used to live in the area but it was many years ago (before children) and there may well have been new attractions.