Look at www.willowfarmholidays.co.uk Ive been to both them and to Bilsdale. Both really nice families who only take small groups so kids dont get left out and are treated as one of the family.
Willow Farm is Lincs Bilsdale is Yorks Both are Approved centres I checked.
Willow Farm in Lincs is worth a look for ideas as they do both sorts.
www.willowfarmholidays.co.uk They do a Pony Club Camp weekend in tents for local kids that sounds like what you are doing, and they do residential riding holidays for a full week too for everyone inclusing foreign kids.
Its not South, more Midlands, but only 2 and a half hrs from London. Willow Farm Riding Holidays are good, and cheaper than in the south so worth driving an extra half hour or so.
My friends kids have been twice and it was only £350ish I think in August. They loved it because the ponies werent...