Cotswold Farm Park is good for children if the weather is nice, great adventure playground, lots to see, lambs to feed, chicks to hold, and that kind of thing. Take a picnic if the weather is nice.
Cotswold Wildlife park also good.
Both options would involve a fair bit of walking though.
Bourton on the Water is a pretty village with a river running through it, quite a few "tourist" attractions such as the Bird Park and the Model Village which are admittedly a bit cheesy but kids seem to like them. Gets very crowded in good weather but if you are here midweek it should be fine. There are some nice cafes and it is all on the flat which would be good for your FIL. There is a big car park by the Bird Park, just follow the signs.
I've never been to the "beach" but have heard good things about it.
A bit sad but we like to go to Dobbie's garden centre and look at the fish in the aquarium section! Possibly not your idea of an exciting day out though...
Sherborne village just off the A40 has a lovely little shop and cafe, and there are beautiful National Trust parklands you can walk in too (again possibly a bit challenging for your FIL....)
Hope the weather is kind to you.