I found the benefit of Mensa to be more for me than my kids. Our "local" group is about an hour and a half from our house and, while we do have a GCC (Gifted Children's Coordinator) now for our group, the kids and the events aren't close enough to home to have built a real community for my girls.
A local member started up a coffee night about a year and a half ago which seems to be drawing a lot of 25-50 y/o people, which has been nice for me. I go to that frequently and have really enjoyed the hang out time and conversations, but unless I had a lot of time and $ to attend the AGs (Annual Gatherings) or events in the big city that is my local group, I can't think of much else I'd be getting out of it. I'd look into your local group, the average age of the members, and whether there are any kid events around you.
Our majority of active members of our local group were senior citizens for some time. It is winding up with more younger and middle aged active members now which, again, is nice. I just haven't met a lot, if any, people with kids in the same age range as mine.
I think for me it will be a sheer social experience, but then again, I'm a real extroverted person...