A nice weekend in Swanage would be very family-friendly.
The 'house wrecks' are the Kyarra and Ceritan (sp?). Absolutely loads of others to dive off of boats leaving the pier.
Portland's great for diving with literally hundreds of superb wrecks available to dive in pretty much all weathers
TBH anywhere along the South Coast has an excellent selection of wrecks, many put there by ze Germans on their previous European tours.
So come on down to the South. It's warmer.