When you make the reservations using your FF miles, CALL and make them with an airline representative. They won't (or shouldn't) charge you the extra talk-to-someone booking fee because you are making reservations for a minor. Have them link you in the computer.
Then also continue to watch any change notices that you get for your reservation as sometimes they can get "unlinked". This happened for our Hawaii flights this summmer (3 tickets from 3 different ff accounts plus one more Cash & Miles). Interestingly, my kids and I stayed together when they juggled our flights, but my husband got "unlinked" and put on another. One call to the airline and a "Hey, those were linked!" and they moved him to our flight without a squawk.
When flying from Germany last summer, my dad was NOT linked with us (and has a different last name then us, obviously) so when our flight was cancelled out of Munich and everyone was put on different flights, they worked hard to keep our 4 immediate family together and we travelled through Paris but he was routed through Amsterdam. No big deal until we almost weren't allowed to board our flight in Paris and nearly had to spend the night there. Ooops!
If something like a cancellation of a flight for mechanical problems or weather happens and you need to be rerouted, IT CAN BE HARD on crowded overseas flights to get 4 seats for a family on the same plane (they got us all onboard a plane, and seated our 2 kids together and my husband and I were scattered throughout the plane). I thought at the time that it would be smart to have a notarized letter for each of us so that if we couldn't get 4 seats on the same flight but could split 2 and 2 on different flights (or 3 and 1) and get home faster, that would be worthwhile. I'm sure if you also showed them your initial itinerary and tickets for the whole family and could show how you were separated that would also work.
Three times on international trips (always on the way home) we have had problems. Once we ended up spending an extra day in Sicily. Having the option and being ready to split up your travel group as necessary to facilitate getting home quicker is smart planning!
