Tickets for son

Hilary wrote:

They won't assume anything. The reservation system won't let you book a
child or infant on their own.

How would the reservations system know it was a child you were booking?
Can you point to a passenger visible airline rule that says I have to
informed them a child is on the ticket?

If they're travelling with an adult in a
different booking the bookings have to be cross-referenced.

Why? If you check in at the kiosk (or online), why would the kiosk
care? I have even had no problem checking in at the counter with 2
unlinked reservations. The website forces me into select adult instead
of child, but that hasn't made difference on any carrier I have done
this on in the past 15 years or so. Sometimes I do it because my entire
itinerary doesn't match theirs. Sometimes I even do it just to grab the
extra FF miles for booking each person separately online. When booking
tickets over the phone for one person, I have never had the airline ask
if the passenger was an adult, unless their was a fare difference. For
intinerary the OP was referring to, the adult fare is the same as the
child.