Who doesn't like diggers?! There won't be many children who will be able to resist the allure of riding in a digger and driving over mud and such. But Diggerland is so much more than that. With over 20 different rides and drives don't think you'll be limited to just diggers! There are dodgems, go-karts, off-road rides, mini Land Rovers, a Skyshuttle and more. My family and I attended for a birthday party which was well organised and fun, and all the children had a great time despite the rain we had that day (although the mud did kind of add to the activity so, wellies at the ready, I would recommend a slightly muddy day!).
Definitely worth an explore and with 3 parks there's lots of choice as to where you can go to visit.
Along with Kent and Devon there is also:
Diggerland Durham (Langley Park, Countyy Durham, DH7 9TT) and Diggerland Yorkshire (Willowbridge Lane, Castleford, West Yorkshire, WF10 5NW)