There is a use trail after Sam Mack. I don't know what it's like right now, but you have to boulder hop across the stream to the left/east. The use trail should be very visible and gets more faint the higher you go. There should be cairns along the way once you get to the moraine field...there may be lots of cairns in different directions. Pick the easiest way for yourself and your party.
The glacier is directly south but you cannot see it from Sam Mack and you probably don't want to scramble up that way. Follow the trail until it disappears then follow the ridge that you see in the topo map where the trail terminates up. You may also access it going up the talus slope from Third Lake but I don't recommend that for your party from your original post.
EDIT: Don't forget to stop by Lon Chaney's cabin.
http://www.paulrwilliamsproject.org/gallery/residence-lon-chaney-sr-john-muir-wilderness-ca/
