Hi Helen,
We only stayed at Henley for a couple of nights and then moved onto Crystal Palace for another couple of nights to visit London. It was the October 1/2 term last year and it was bitterly cold (so cold in fact that our car was iced over where the rain/sleet had frozen overnight). The CC site from recollection was a very basic site - well kept, good shower/toilet block etc, no play area that I can remember. It just off the main road so you have the traffic noise plus intermittent Heathrow aircraft noise. I would say that as a base to explore the area its fine but if you wanted to stay just at the sight my three children would become bored pretty quick.
We're actually going back to Legoland in Sept but we are going to stay in a hotel in Windsor as we want to also spend a day exploring Windsor - the kids love castles. We only stayed at Henley last year as we combined Legoland with visiting the London museums by basing ourselves at Crystal Palace CC site (which incidentally I would not do again as bus/tube hopping in and out of Central London with three kids under 8 is hard work. But at least with can say that we tried it!).
Apologies for being a bit long winded but I got there in the end!
Cheers,
Mathew