I went to Chessington last year (after a break of nearly 6 years) and must say I was quite impressed. They seem to have done a lot of work and were still working on a new area that should be open by now. It was aimed more at families with children than it used to be and I had a really good day out with DD (aged 9).
I went to a Gullivers Kingdom (but I can't remember where - Peak District I think) in 1999 and was very disappointed. It was aimed for very small children and was quite shabby. But it might have improved over the years.
Not been to Oakwood.
Libby