Have been meaning to post for a while and mention this -it's a newly opened park ( last autumn but is still under development) that is ideal for under 12s - dd is 3 and just loves it - we bought a year's pass when they were just £20 and go regularly. It's a bit wind swept at the moment as it's on an exposed hill side - but as the weather gets better it'd be ideal for a day out with children.
see www.twinlakespark.co.uk
( sorry - have lost my written details on how to make a live link and have forgotten how as I've been away so long!)