Thanks for the responses. My bargaining power here is weak, since this is a redemption booking using BD miles. So highly unlikely that I'd be able to switch to another carrier at such a late stage.

I'd guessed the automatic seat allocation system would try to group children and parents, but the complication is that all four of our tickets are adult ones. No idea, in these circumstances, whether the seat allocation algorithm is sufficiently sophisticated to identify children based on entered birth dates.