Matt, we just got back from Chamonix with our family of 4 and had a fantastic time. It was by far our favorite part of our France trip. The multipass worked out great and I would definitely recommend it. The one glitch we experienced that I can advise you on is that I misunderstood where to pick it up. I did buy it online in advance, and got the same price (second child free). While online I also made a reservation for the Aguille du Midi for our first morning (yes, you have to make the reservation separate but there is no fee if you buy the multipass first). When we went to the Ag du Midi station, we found out that you must pick up the multipasses at the rail station instead. This is in the email, I just didn't understand it as this was our first time in Chamonix! So we had to run around town a bit and missed our reservation for the Ag du Midi for that day. Happily we went early the next morning (7:30) without a reservation and had no problems at all.
My recommendation would be to buy it online in advance, pick it up at the Montenvers station on Weds morning, and then take whichever rides you are planning. When we got to the office at Montenvers they had a package waiting for us with our name on it, picking it up took less than a minute. As noted in the guidebook, it's really nice to have the multipass rather than to wait in line for each ride. The package also includes a buy-one-get-one-free discount for the alpine slide (we got 4 for the 4 passes and used them all).
I will also share that our family's favorite part was the ride up to Plan Praz and Montenvers, even more than the Aguille du Midi. The views are fantastic from that side of the valley. We hiked RS hike #2 (Grand Balcon Sud) and with our younger one (a short 9 year old) and me moving at a slower speed, it took 3.5 hours (instead of the 2 hours listed in the guidebook). It was an amazing hike but definitely longer than we expected. We didn't do the hike on the other side of the valley (I believe Plan du Midi to Montenvers) the next day.
