Like everyone else, I don't have all the facts, but am tying to get it to work for the next family. Also, the child in the car seat was 1, not 2.

1. Buy the 18 year old's ticket without using a frequent flyer account, and as a guest online with an address different from the first ticket (if necessary? not sure). Then no way the airline will cancel the duplicate reservation.

2. Make sure the 18 year old goes thru security twice, once with each boarding pass, so they both get scanned by TSA.

3. DON'T tell the gate attendant what you are doing! That for sure will cause the whole thing to collapse.

Then I think you are good to go.