If the flight was full, it could have cost DL now up to $10,000 to do a VDB for the seat that the lap infant occupied. If the flight wasn't full, people with lap kids can use otherwise unoccupied seats for the kid for free.

I wonder whether the older son's boarding pass actually boarded the flight or not. It might have been easy to just use it for the lap kid and probably the DL FAs and GAs wouldn't have noticed unless their software had already caught the duplicate booking and canceled one or both flights for the older kid.

ADDED: Attempted theft of services.