Legoland Windsor and Chessington

[Squaddielife]

Hiya
We're (mum/dad/8yr old DS) first time merlin pass holders and are off to Legoland Windsor and Chessington over the easter school holidays/bank holiday weekend.
I'm assuming it's going to absolutely manic!!
With this in mind do you have any tips to maximise our time?
We have 4 days. 2 in each I think although it's up to us.
Want to do Chessington Zoo too - is this just a normal zoo experience?? eg stroll around at your leisure?
Can anyone explain the qbot and fast passes to me? Are both available at both or is it qbot for LL and FP for chess?
Thanks in advance

Handy thread here
www.mumsnet.com/Talk/days_out_recommendations/2861243-The-New-shiny-LEGOLAND-2017-hints-tips-and-reviews-thread-Come-on-in
and last year's
www.mumsnet.com/Talk/days_out_recommendations/2605780-THE-LEGOLAND-2016-Tips-Hints-Reviews-and-Questions-answered-here

The zoo at Chessington is part of the park so easy to dip in and out. We were really impressed with the zoo,including the aquarium. If you download the app it tells you how long the queue for each ride is,however I think the timings are over estimated so we were quite surprised!
Can't comment yet on legoland yet as we haven't been,we're going in May.