Speedwell is good – nice underground boat journey – I loved it as a kid.
I think they are all basically a short guided tour, something like 45 minutes, probably the right sort of length for a kid.
Not good if he’s scared of the dark – you can’t just take a scared kid back out when you are 15 minutes of boat journey + steps away from the start, you have to wait for the tour to finish.
The Devils Arse is supposed to be good if you like League of Gentlemen as it is just like the sketch from there, but possibly not so good otherwise.
Lots of nice walks round there too, hills to run up and down etc.
The castle is a good ruined one, you can clamber up bits of it, but possibly a bit of a waste of money paying to take a 3.5yr old in though, as they’d probably be just as happy running up and down the hill round the corner.
Joe