my dd (6) loves chessington, we live 30 minutes away so have been a few times, we usually get there when it opens and leave as they close. We have never stayed at the hotel, but it looks ok (it overlooks the Africa bit of the park). We haven't been to lego land yet so can't tell you if it is better, but from the days we have had there i would highly recomend chessington
I've been to Chessington lots and it has lots on offer for children aged say 5-Teen. There may be some rides he can't go on - so do check out ride height restrictions on their website. Rides I can think of which he might not be able to do are Dragons Fury (min 1.2m) and possibly Dragon Falls (which has increased height requirement recently to 1.2m). There are also a couple which are 1.4m, such as Rameses and Rattlesnake.
I've never stayed on site at Chessington due to living quite close but have stayed at Alton Towers and it is quite nice staying very close to the park.
We went to Chessington in April ( it was Easter or the Royal Wedding w/end IIRC) because we had Merlin cards which ran out end of May. Didn't have to queue to get in The ride times queues were over an hour for Cobra ride, Vampire ride . I think we went on 4 rides with all the waiting. (We should've bought the Q- Busters in retrospect) Did try to get a special deal at hotel ( they do a certain number of Merlin card discounts on rooms) but had none left.
Just recently been to Chessington with two under 4s. They both had a fab time - so did I! There were tons of rides they could go on, and they absolutely loved it! Went on a weekday, so hardly had to queue for anything - but some of the bigger ones had long queues (we obviously didn't go on those). The zoo has some nice bits, too. We especially liked the walk-through monkey area!
You can easily walk from the train station, it's only about a 10 minute walk, maybe 15 if you've got some short-legged, tired people with you.