Not sure if you got this answered. I had the same problem.
1) Pay for the child seperately. (which it looks like you've done).
2) Pay for yours (parents) travel.
3) Then, and this is the part that messes most people up. When you list for the flight on the "ticket input" page, list yourself using the ticket number, and then select "add" under ticket number and list your 2 yr old using their ticket number.

[email protected] if you need further help with this, they usually respond in minutes.

Hope this helps