I would have guess that upgraded lap childs don't happen all that much. I assume it's uncommon for a lap child to be traveling with a single parent (of course it happens, I just think it would be uncommon). So for the lap child to be upgraded, that means the parents either (1) split up, (2) both are elite and both get upgraded, or (3) get upgraded as an elite and companion. #2 and #3 probably don't happen that much unless one/both are Platinum, and the risk of being split up probably discourages most couples with lap child from even standing by for the upgrade in the first place. So the parents would have to be willing to be split up. And I guess some are. Which I find surprising.