Hotel would be IMHO by far the best option - means your wife and son can have a sleep and a bath/shower and something to eat in a relaxed way before the onward flight.
Am not sure visiting any sort of tourist attraction after a TATL is a good idea for anyone !
In any case Windsor Castle has varying 'last entry' times depending on time of year - and is 3.00 PM Nov to Feb and 4.00PM rest of the year so might not have enough time to get there after the flight arrival (you don't say when your wife and son are travelling) and then you would only have an hour or so at most before the castle closes
http://www.royalcollection.org.uk/vi...lan-your-visit
Same with legoland - looks like it's closed November to March
http://www.legoland.co.uk/Plan/times...h=11&year=2013
plus I was
shocked at the price of admission - £35 adult and £ 30 for child for discounted online ticket !