In the course of googling to answer some questions in the thread about the Mom of Disabled child thread, I found this handy dandy chart which states how much airlines charge if you want an additional seat for a child under 2 on an international flight. For all I know, this may be outdated, but
Viva Air France!!!
http://lifehacker.com/this-infographic-tells-you-each-airlines-rules-for-flyi-1637935173