When you book on delta.dumb, you can usually select seats during the process rather than after buying the tickets. This can be used to "guarantee" your seats. Or call DL and book the tickets by phone, even after you've found what you want by looking at schedules on delta.dumb. The phone agent can grab seats for you during the booking process.

Depending on the child's age, the GA will find seats together for you or you can get separate but desirable seats and then ask other passengers to swap. Or you can pay EC fees to guarantee getting good seats together.