We have just come back from a day at Magna and we all had a great time. Dd (7) had a go on the forensic science workshop and ds (4) had a wonderful time in the Earth pavilion blowing things up and controling real diggers. Lots to do and all hands on. The staff are very helpful.
Only down side is that the tables in cafe area were rather grubby and sticky!
It isn't cheap, as a family ticket will set you back £28, but we had a wonderful time and spent a very easy 4 hours there. Access might be difficult in some areas if you had mobility problems, and it is dark with lots of flashes and bang so no so good for nervous children. There are also flashing stobe lights that might be a problem for some epileptics.