I had the same problem trying to book an actual seat for my 10 month old using Avios on AA for LAX-OGG. I got around the website by adding two years to my child's birthdate. The website has no problem issuing a child ticket for 2+ years old. I'm assuming I will have no problem at the gate with this. (crossing fingers). The thought of having to deal with the BA call center was daunting.

Now that I see this part about the child fare, is it any cheaper number of miles?