it has to be castlerigg and long meg as everyone says, just because not only are they WOW! but very accessible, car wise, walking wise, etc.
if that sets them off i can't recommend swinside (sunkenkirk) highly enough...bit more of a walk, but hey! walking is great for families (just do what i do and don't mention theres any stones on the way then HEY! fancy that look whats here! groans of 'we've been had' follow but too late! the deed is done!) its an amazing site, really really worth it.
just enjoy, and let them do whatever makes em happy
my 12 year old brings his binoculars and bird watches and 9 year old brings her sketch pad and just chills out or races round like thing possessed!) but have to say it tends to be a non-family orientated pastime !!

hope you enjoy it all!
to the stones!
pixie
xx