The Guggenheim Museum has a great set of guided family programs include Second Sundays interactive gallery tours, Family Tour & Workshops, Family Fun for Family Members, Family Days, and their Just Drop In! program. There are also other self-guided options when visiting on your own. Pay What You Wish: Saturdays from 5:45 p.m. to 7:45 p.m.
My girls (12 and 8 at the time) and I spent the morning at the Guggenheim. I had read somewhere that the front desk had "activity packs" for kids and that made all the difference for us. My 8yo loved the different spotting games and activities in the pack (had to leave my DL as collateral). The architecture of the building fascinated us all too.
The Guggenheim is wonderful for adults, teenagers, and some middle schoolers. The paintings are simply amazing to look at. Not sure that all middle-schoolers would enjoy this. The museum is set up so that you walk up and around in a cylinder to the top floor. It takes a few hours to go through. I would definitely discourage children under 7 from coming as this would not keep their attention.
I recently went with my kids to the Guggenheim Museum and I have to say it was pretty breathtaking. It was a great place to see, although I must say that I can't imagine it would be fun to go here more than just a few times. The kids may become a little bored, and this may be more interesting for the parents, instead of the kids. I'd recommend that you give it a try though.