Firstly I'm sure this dupilcates threads previously discussed, but I couldn't find any after much trying, so apologies.

Background: I booked ticket for my wife and I to fly back from Palma Mallorca to Heathrow last Sunday. I later rang BA to add our 18 month old son. All fine in MMB, APIs entered, seats selected etc., but couldn't check in online, which I've found to be a common issue travelling with a lap child so thought nothing of it. At airport check-in, we are told my son has not been issued with an ticket number, and to call BA to sort this. My heart sank as normally I check we all have ticket numbers (I suspect already putting me in the very small minority of travellers!), but this time I just didn't get around to it. After 45mins of holding for BA on the Gold line, the ticket was issued, but 10 minutes after check-in closed. BA put us on a flight the next day (sadly now amber instead of green) and offered to pay for a hotel for the night.

I have a memory of reading somewhere on here that technically it is the passenger's responsiblity to check a ticket number has been issued. Is that correct, as I couldn't find anything to that effect in the general conditions of carriage? If that is the case, fair enough, although it seems a little unfair as I strongly suspect the vast majority of travellers won't have any idea of the difference between a ticket number and a PNR which says your flights are confirmed. I appreciate BA's upfront offer to pay for my hotel - is that just a gesture of goodwill that I should be grateful for, or part of any legal requirements, eg. is this involuntarily denied boarding (I suspect not, given my son didn't technically have a valid ticket)? Many thanks all.