I don't have kids, so for me I equate trying to get in kids for free to trying to get kids on the upgrade list for free. Both times the Medallion can say "they won't drink alcohol in the SC / in F so it's not much cost to you." But still, First class is clearly a zero-sum upgrade game. The SC is also, but to a much lesser extent since it takes a lot more people to fill a SC. Still, waived kids could mean other people could be denied entry later due to crowding.