I would love to sit next to some random 7-year old kid. Assuming s/he is well-behaved and mostly self-sufficient. Why? More elbow room for me!

One potential solution is for the airline to automatically assign seats at the time of booking when there are children on the reservation. But the passengers cannot change the seats. Maybe DL doesn't even show them which seats until OLCI. And maybe the algorithm purposefully splits up the adults (e.g., one adult with each child) out of spite.
And if there are no adjacent seats available, then there's an extra warning or it prohibits the sale altogether.