Ok then...Zoo is fab but pricey
Depending on age of kids, SS Great Britain may be worth a poke around...surprising engaging for children actually
Lots of good soft plays with cafes for grown ups; which one depends on where you will be based
If you fancy a drive and are of a hardy disposition (its cold there!) then Slimbridge has lots of ducks and geese to hand feed. Not everyones cup of tea but we like it!
Ditto Westonbirt Arboretum - has a winter lights thing on at the mo I believe
There are a number of city farms that would warrant a quick visit, again depending on childrens ages and your location
Some of the local authority swimming pools have nice kiddy pools
Damn. Need to think..may pop back later...!