I haven't taken kids to Yellowstone, but I wouldn't do it at those ages. There's a LOT of driving because of the distances between airports/hotels/park and the size of the park. That would be a deal breaker for me.
Also Yellowstone is kind of dangerous for people who don't follow rules well (which would be my kids!). Between wild animals, getting lost and the geothermal features (my biggest worry at Yellowstone), I wouldn't take kids under age 5 or so because I would be too worried to have a good time. But I am sure people go and have fun and nothing bad happens so that is probably just me being paranoid--and having super adventuresome kids.
Catherine