Farming World at Faversham is great - indoor play area/painting/massive outdoor play area/crazy golf/lots of animals.
If you don't mind going a bit further, you've got Howletts/Port Lympne. Or even a bit further still is Colchester Zoo. I went there last week on a trip with school and will definately be heading back there with Cara. I got to feed a giraffe!
Museum of Kent Life is nice, but it's not as big as Farming World. They have an indoor play area, an outdoor one and some animals.
Diggerland? Not been here, so can't say what it is like (despite it being 5 mins from me!)
Leeds castle.
Hope this helps a little and you have a great day where ever you end up.