Yes there are kids clubs, they seem to be well run, but they are expensive. TBH the whole place is fairly child oriented, so other than taking a well earned break at the spa, even if you do put them in the kids club there isn't a lot to do where youwon't bump into thousands of OPKs (other peoples kids)
We went to Elveden - kids club was brilliant, staff were fab. Not sure the 12 year old would be up for it - in fact I'm not even sure they go up to that age - there are so many other activities for a 12 year old to do.
Given the children's ages... I wouldn't bother. The 12 year old is too old for Time Out Club (max age is 7, up to 8th Birthday). In my view the activities in general are aimed at young children. Spend the time together as a family - do the adults need free time? Could one adult get free time whilst the other does an activity with the children? Children don't need lots of activities planned - you may find that you go swimming quite a lot. The environment is also quite safe, so you may find that your 12 year old may want to go off alone for a bike ride for example. If the adults fancy a spa evening, then Longleat provide a babysitting service.