I'm looking at flights sold by UA but operated by AC. I've been warned that I won't be able to reserve seats at the time of booking.

I encountered this situation when buying a flight from DL that was operated by AS, but by contacting DL I was able to get my AS PNR and use that to reserve my seats on the AS segments.

Is it possible to do something like this to reserve in advance on the AC segments? I'm travelling with small children and do not want to leave seat assignments until the gate.

I'd also consider buying premium economy or extra legroom seats, if AC has such a thing. If they do, can I buy from UA at the time of reservation or should I deal with AC directly, and will that get complex (i.e. high likelihood that it gets screwed up)?

Thanks!