Try to spend most of Saturday up high to get the boys acclimated -- Horseshoe Meadows is the closest.
At Whitney Portal, you might walk up to the first (and the only major stream crossing) to see how bad it is. If you try crossing it as practice, you could then come down the old trail (just across the stream, look for it on the left.) If you used that in the actual climb, you could skip the crossing.
Take layers of clothing. Use sunscreen. Snack and drink water often.