If you have status it's probably worth a call to the EC as they may be able to help.

The other, non mutually exclusive option is in fact to ask REALLY nicely when you board, if the CC can ask the other passengers if they'd mind a change and explain the reasons.

What I suspect you can't do is INSIST on anything. The bottom line is you're flying in economy, and they have fulfilled the primary obligation which is to seat one of the parents at a bassinet station.

I can take a look on expertflyer if you want and check the current state of affairs re seating. Just need exact dates and flight numbers.