this place is nice if you get a nice day - there are animals and a little train and indoor soft play as well as outdoor playground. It's between BAth and Bristol.
Victoria Park in Bath itself has a fab playground and is free (!) - good place to take the children to burn off some energy if you've done a bit of sightseeing.
I'll try to think of some more. I think the public swimming pool has waterslides now if the weather is bad and you need an indoor activity.