Thank you for all the great tips and ideas, lots to think about.
I have definitely looked into the Fishbourne palace. Definitely an option along with St. Albans and Bignor Villa. Want to go to one of those three . However, too much history and they will be bored. St Albans is easier to get to and have some great programs for the kids to work with as well. Bignor would be my favorite stop, but hardest to get to (although looking at the flex bus in the area might be a way to go if we get to the train station near it). Could also do Fishbourne and then Continue on to Portsmouth for the afternoon and evening.
As someone mentioned the White Cliffs are not part of the theme, however my last group I brought that was the one place they actually wanted to stay longer at (they were a whiny group and complained at everything!) and the one place they kept asking to go was the beach! Even if not pretty it might suffice for a few hours, but will look into the other options mentioned
I think as James mentioned, would love to stop in Salisbury and see the cathedral. If we have a coach and enough time that will be exactly what we do.
Will be most likely taking train to Dover and then taxi up the few miles to the cliffs. Probably need another quick taxi ride if we go to the castle as there seems to be no quick walk to the castle despite it not being that far away.
Thank you for all the ideas. I am sure I will be back with more questions on another topic soon!