Delta thinks an 11-year-old (or a 6-year-old for that matter) is old enough to sit alone in the same cabin as a parent, but not old enough to sit in a different cabin.

And yes, the logistics of the upgrades and how they affect seating are among the perils of separate PNRs. I don't think parents should have to give up all chance of an upgrade for themselves and their child from the outset, but they do have to monitor the situation and cut bait early enough to get seats together in coach if it looks like both upgrades won't come through.