Hi
I am not sure where you live but there is also Paradise Park which is up the A10 in Hertfordshire. I have a couple of young sons and I have taken them there on a few occasion they really enjoy it. It is probably cheaper than Chessington but they still have quite a varied animal section, some free rides, a Thomas the Tank Engine Railway (£1 a go, I think), various playgrounds, forest walks and a swimming pool. I like it because it isn't too big but there is enough to keep the little ones occupied for the whole day.