went to @Bristol (science bit) with a 1yr old and a 3yr old (only one was mine ) and they both loved it. There was plenty for them to be interested in. Although they didn't really 'get' the science they both loved it.
NOah's Ark is also supposed to be good. Also Bristol Zoo. But @Bristol is central and easy to get too. You could maybe do wildwalk and science bit in same day (there's a ticket you can get to do both), but it depends how tired the kids get.
HTH