Two different answers for our two posters.
To get to the Contemporary, I think your idea is a good one. However, if you're going before the opening hours of the MK, the MK busses are specially intended for character meals, and you may be able to be dropped directly at the Contemporary if you let the driver know that's your destination.
If not, the walk from the MK bus depot to the CR is a short one, if the weather is nice and your kids aren't too small.
For the second guest, since you want to use your own car, the simple thing to do is to drive to the MK lot, take the parking lot tram to the TTC, and just walk to the Poly, which is right next door. (Depending on the time of your reservation, you might also be able to take the resort monorail, but walking might still be faster.)
Hope these are helpful.