Do not risk adding an infant at the airport if you're flying to/from Canada. It has been said on this site and elsewhere that you can simply show up and they will add the infant for you. Technically, that is true but that doesn't mean they're happy about it.
I recently flew YYZ>LGA and back on two separate one-way tickets. Both times I did not put our infant on the ticket to save the 900 points each way. In Toronto, the agent (on priority line) insisted it is BA's problem and would not proceed until I called them. After waiting on hold for 20 minutes (with a crying infant naturally), I was told that because the flight was less than two hours out she is simply unable to add it. At that point, the agent grumpily gave in and created the ticket for $17 in taxes.
On the return, I got the same response but the agent did not insist I call BA, but instead got on the phone with someone for over 30 minutes to create the ticket. Apparently because its international the name needed to be added to the manifest which was something he and the other agent claimed they could not do on their own. I will not pretend to understand what exactly the problem is, but suffice it to say it was not worth the 1800 points saved round-trip.
I would have been much happier to have fully confirmed tickets in advance than to duel it out withe the front-line agents to save a relatively measly 1800 points. From what they told me, on a straight-up domestic flight it would have been no problem.