I highly doubt they will let the two kids in on single pass (as opposed to club member or holding status with airline). I think plan for that not happening and be pleasantly surprised if they do.

I’ve flown first class with my baby twice. Flight attendants very helpful- they offered the bassinet for the baby or their own seat (obviously in bed mode) so the bassinet wouldn’t impinge on my own space. As she wasn’t old enough to roll I elected the second option.

Be careful with Singapore airlines award booking with an infant- Each infant will be charged at 10% of the revenue fare. Ie if revenue price is $5000 return you will be paying $500 for your child to sit on your lap!