Welcome to FT.

Whilst I understand the sentiment as a parent myself, most likely you don't have to rely on understanding of someone else as the flight crew will be, as a last resort, sort this out for you. When check in starts the flight is, I believe, under 'airport control' so customer services can't be of help.

Just tell the IFM your issue once you board the aircraft. And yes, claim back the seat reservation after the flight.