A plane has at least a lot of people around. In postal service, one can assume the kid spends quite some time alone with the operator. You just have to trust that random person!

Going through the postal service might have been safer than other forms of travel because there would have been more explicit exchanges of custody. Like put a kid on a passenger train and they could just get off and get lost before getting on a connecting train and who knows what happened to them? They arrived on train A but never got on train B.

Whereas with mail, the kid becomes the operator's responsibility. If the kid is handed over to the operator to ride train A but then is nowhere to be found at the destination, then that operator is the prime suspect. Or the operator hands the kid off to the next operator in the journey and the kid is now their responsibility.

And even if you have a babysitter, passenger stations are going to be a lot more crowded than cargo stations, so keeping track of a kid will be a lot easier, plus the driver is the babysitter for the postal service, so they won't miss their train or take the wrong one by accident (unless they are routed incorrectly lol).