I think the Delta staff erred in many ways.
I am wondering though what the gate staff would have done if the parents, before boarding, told Delta their ticketed oldest son would not be boarding, due to other plans, and what they - the parents, expected to do with his reserved seat.
Would they insist, due to regulations or their rules, that no matter that THEY had paid for the seat, it had to be occupied by the passenger named on the ticket? Would they said that and let them know before boarding they’d have to stick to their original plan of seating the younger children in their laps, as they expected to do originally? And then would the whole mess have been avoided? Maybe.
Regardless, the behavior of the Delta staff, threatening jail and parental loss of the children was atrocious.
“I am wondering though what the gate staff would have done if the parents, before boarding, told Delta their ticketed oldest son would not be boarding, due to other plans, and what they - the parents, expected to do with his reserved seat.”
They did tell the gate staff. Gate staff said, “Fine.”