I live so close to the hop farm I get half price membership - many changes afoot their though.
Bewl water
There is a more child friendly climbing place near T.Wells called Evolution which is fab
Broadstairs/Whitstable are fab but from you quite a trek - Whitstable is around 1 hour and BS 20 mins+ longer, BS has THE BEST ice cream parlour in the world but DO NOT under any circumstances eat fish and chips from the place on the sea wall.
Tyland Barn is great with pond dipping and wildlife based activities either free or for tiny £
Museum of Kent Life We love it here and often get the train from Paddock wood and then go by boat from Maidstone, it is a wonderful day out. Also if you 'do' gift aid you can enter for a year on your ticket.
Leeds Castle has a fab new playground and you can also visit all year on one admission ticket.
Howlets Zoo only took 55 mins from us and was a good, if pricey day out - as all Zoos are. Check the press releases on their website, sometimes they have 'kids go for £1' days.
I have more but I've got to go know!