BA is quite good in that , if the journey commences whilst aged under 2 , the infant fare will be charged for the entire journey - even where the infant turns 2 during the trip . ( see
http://www.britishairways.com/en-gb/...-travel--index )
Emirates will charge infant fare for portion of journey where under 2 and child fare once the child turns 2 ( see
http://www.emirates.com/english/help...rs-on-emirates )
Both Air France and Lufthansa will charge the child fare for the entire trip (
http://www.lufthansa.com/uk/en/Flying-with-baby and
http://www.airfrance.co.uk/GB/en/com...er_enfants.htm )
If BA's fares are not much more than the others, then only paying 10% for the entire trip may make BA work out best value given that the child fares are not much less than adult fares ; child has 25% discount on the base fare, but all taxes and carrier surcharges to pay )
Looking at FRA-DXB as an example , taking lowest fares
BA adult fare would be €442.54 with infant fare of €109.38 = €551.92
EK adult fare would be €501.16 with the mixed fare being approx ( €461.16 + €17.00 ) / 2 = €740.24
LH would be €460.16 + €428.16 = €888.32
AF would be €483.59 + €434.59 = €918.18
Although BA's adult fare is 2nd highest , it easily comes out cheapest for 1 adult and 1 infant/child
This does seem to be a question that would be best served in the travelling with children forum