We were here in July 2012 (mom and dad, 40s; two kiddos, ages 7 &12; and grandma, 70s). We thought this was in the top two visitor's centers here in the park with the other one being in the Canyon/Falls area.
This was a fun experience for the kiddos while they were waiting for Old Faithful to erupt. There are lots of hands-on things to enjoy here and lots of learning to be had in a way that only seems like fun for the kids. Our two boys really enjoyed this place.
You can also pick up the papers here for your kiddos to become junior rangers, although you can also get this information beforehand from the Yellowstone website, which we did, so we had ours ready to start before we even entered the park. It really helped our boys to appreciate the area by having them find things along the way, and they learned a lot more than they would have had they not done it. They also get a badge showing they are junior rangers when they are done. :)
Make sure you slot in some time to check out Old Faithful Inn also and enjoy some ice cream there while people watching. Great area that we highly recommend!!!