We fly SW almost exclusively and that far back in B, I’d say it’s 50/50 as to whether you could get a full row of empty seats. That said, there’s a decent chance you could get 2 together on one side of the row and a single on the other side of the same row.
You might try booking early bird boarding for just one of the adults. Then that person could at least save a middle seat for the child (no one’s going to want the middle seat that early). It would only cost $25 extra each way to do that. That person would board first, then the other adult boards later with the child, drops the child off with the previously boarded adult, and looks for a single seat nearby. If the first person picks a row in the back, you might get lucky and end up together.