Nothing wrong with that.
There are other options too. Why not go someplace like Aquatopia at Camelback? Grandma can play with the three year old at the waterpark while you all ski, then meet up for dinner. The resort offers things like arts and crafts, magicians and wildlife shows in the lobby. Or you could ski in the morning at Camelback (which has a daycare for 3 year olds btw) and stay at the Great Wolf nearby, so the whole family can enjoy the waterpark together in the afternoon after morning ski camp.