Hi Rich
I assume you will be travelling down here by car.
Hastings is an historic town but it does have some interest for the kids. Once you have exhausted the facilities at the Haven site a trip into the old town of Hastings is worth while. These places would interest the family: see
Discover Hastings - Great family attractions in 1066 country
A short trip up the road to Battle would enable you to visit the place where the English got a bloody nose by the French in 1066: see
1066 Battle of Hastings, Abbey and Battlefield : East Sussex : South East : Find a property by map : Properties : Days Out & Events : English Heritage
This might be a little dry for the kids but the street cred they would get from being where the English had a good hiding might swing the day.
On Wednesday you could then head out to Herstmonceux for the afternoon and visit the old Royal Greenwich Observatory, now a science centre: See
The Observatory Science Centre
This is a super place and there are lots of hands-on things to do.
By arranging your visit carefully you could then make your way into the village and find the Woolpack, have some grub maybe and then the Coasters will turn up for their Wednesday meeting
and it would be great to see you and the family.
If you decide to have a trip to Brighton the 'Cannonball Run' finishes there on the Friday: see
The Cannonball Run Europe 2009 - What's On - Surrey, Sussex, Kent, Hampshire, Berkshire, Buckinghamshire, Oxon, Isle of W - What's On July – What's On this Weekend, July What's On, July Events, Events in July South What's on, things to do, places to
Apart from that there are stately homes and castles but I dont think you will have time for that.
Maybe a meet up in a pub for a bit of banter (posh word for bollox) would work out ok during your week.