I believe Delta’s only policy regarding small children is that they must be seated in the same cabin as their guardians. They can, and occasionally will, be split from their guardians. I can think of two instances off the top of my head where my four-year old and my wife were split, once because of upgrades (she forgot to unselect “request upgrade” and they were upgraded into different rows) and once because of an equipment swap. The former was cured through a seat swap onboard; I believe the later was addressed by the GA at my wife’s request before boarding started. But, splits can definitely happen.

I also sat next to a 7 y/o once while his mom sat in the same row across the aisle. They both wanted window seats.