Yes, Yellowstone will be crowded but with children that's the time families have available.
You can definitely do all three. We've done it when the kids were small and we tented the whole trip. :) If that's all the time you have then so be it. At least you'll get to experience the parks rather than not seeing them at all.
I'd recommend staying on the west side of Glacier for more activities for the children. The east side doesn't offer much outside the park. However, being IN the park itself is an added plus.
For Yellowstone - again, staying in the park saves a lot of driving time. Fishing Bridge is the only place with hookups in the park.
Don't try and do Yellowstone and Grand Teton from one spot. You certainly wouldn't be driving it more than once as it covers a great distance with slow driving.
Move down to Teton Nat'l Park and stay at Colter Bay RV.
If you spend 3 nights at each spot you'll get an overview of the parks. You'll want to come back for more, for sure. Then three hard days of driving back and forth.
