Help please - flying in a few hours and seated away from toddler

We are currently on a trip to Tokyo with my toddler, and due to a family member falling critically ill, we had to cut short our trip and fly out on the next available flight. However, we did not manage to reserve seats together when changing our flights, and are now scattered.

The worst part is that our 2-year old will have to sit alone - the bassinet seat, which is usually held back from being booked, has been taken.

We have offered to pay to upgrade to business class or premium economy, but due to the sakura season, the flight is pretty full.

We are dreading tantrums / meltdowns, and seriously no toddler should be flying alone without a guardian. We are not sure how the booking went through when there's a minor reflected, and only single seats are available.

We are hoping to swap on board / hoping a seat frees up near him. Hoping to also bring along some extra Tokyo snacks as a big thank you for someone who can swap with us... Going to try going early to the check-in counter and boarding gate agent to beg for information re: allocation. Any tips on how to get seated next to our toddler within such a short time please?

TIA!

Edit: Thanks everyone for your responses! Quick update - like many of you said, the check-in agent and boarding gate agent have the most visibility on seating, and the check-in agent was able to make changes so our toddler was not sitting alone. We headed to the airport about an hour earlier to explain the situation at the check-in counter, and it was quickly resolved.

Thankfully, it was not as dramatic as some of you had feared. There were some tour groups on the flight with bulk bookings, which also meant that there was flexibility within these groups to make changes, and I think that was where the check-in agent had focused. No drama on the plane, or at boarding. Everyone was happy as a clam, and the gifts we got (alcohol, snacks and make-up like some of you here suggested) became gifts on standby in case our toddler was having a bad time on the plane.

The family member who was critically ill unfortunately passed away. To those wondering if we could have waited to take a different flight, no, unfortunately said family member only had days if not hours to live, and it was for us to rush back to say our final goodbyes.