As you noticed the seat reservation process blocks this, in order to stay within AC's policy (which, if they filed with CAA they must follow by law).

Bypassing this is risky. You can end up booted from exit row in favor of a random midlde seat in the back.

It would normally get caught:

-If you have any interaction with checkin agents

-If you have to show passports they will likely look up by last name and see your whole family at once.

-It you board at the same time they will see an exit row
prompt saying you need to be free of responsibilities for other people

-If you DO board successfully it will often fail if you plan on sitting the rest of the family NEAR the exit rows and talk to / help stow bags / visit with / acknowledge then you during boarding or before takoff.

carriers interpret the responsibility part as applying to all parents/guardians on the ticket rather than just one parent next to the child. I've been told this is since parent might delay egress by seeking out their kids in different rows of the plane).

On the US side of the rules:

FAA does not distinguish what they mean by 'small' children so airlines interpret it by anyone who would need an unaccompanied minor fee : FAR Part 121 Sec. 121.585

"
(b) No certificate holder may seat a person in a seat affected by this section if the certificate holder determines that it is likely that the person would be unable to perform one or more of the applicable functions listed in paragraph (d) of this section because--
[...]
(7) The person has:
(i) A condition or responsibilities, such as caring for small children, that might prevent the person from performing one or more of the applicable functions listed in paragraph (d) of this section