My kids flew UM several times.
You go with them to check in where they get a plastic envelope with all their documents to hang around their neck.
Then you go through security and sit with them at the departure gate until a member of the flight crew comes to get them. You watch the kid walk with the flight attendant into the jetway.
So I don't understand how the adult at the departure airport let that happen, unless the airline takes the kids right from check in the DR.