There is a glitch on the website were you can take "as many" kids free for this great day out.
Normally you can get 2 under 16s tickets for each adult ticket bought.
However if you buy 4 adult tickets and put 8 free kids tickets and then at the checkout change the adult amount of tickets down to 1, the kids number stays at 8 and it goes through fine....
(you could do this and take 30 kids free if you wanted)
I go every year and its a great day out!!!!