Boarding a plane without that guarantee and then relying on somebody else giving up their seat to accomodate me does not seem like the thing to do.
The airlines should not even board passengers with small children unless it has been confirmed at the gate that their boarding passes are for adjoining seats.
It’s happened to us (wife, daughter (aged 4-9), and myself) probably half a dozen times where we were upgraded to business class on a transpacific flight at the last minute and switched once we got on the plane. There has never been an issue as every seat is an aisle and there are a lot of single travelers that easily switch.
It also happened to us where they switched to a different aircraft causing seat changes, again there was no hassle to change with people.
Surprisingly we have never had the issue in coach as we pre-booked the seats.
'Reserving' seats doesn't seem to make any difference. Back in 2019 my wife and I flew Lufthansa from Houston to Frankfort, Germany, and then on to Budapest.
Coming home we flew American from London to Houston.
We did not receive any of the seats we had selected during booking. That all changed when we checked in at the gate. I think at that point we were lucky that we just got to sit together at all.