My understanding (having read many threads, NOT based on personal experience) is that the airline policies are based on the airline booking the award travel.

However, even "10% of ticket" can vary wildly. Since you're not paying cash, which ticket will they use 10% of- the cheapest one available at booking, full fare, or...? I have also read of cases where it's 10% of base fare + all applicable taxes, fees, and fuel surcharges (i.e. that can be almost the same as paying for a ticket!).

BA seems to be the friendliest, since as I understand it, you can pay the 10% infant fare IN MILES (i.e. 10% of whatever you paid in miles). Not sure what happens with taxes/fees/etc then, esp. since BA tends to have a lot of those. But if you can use BA miles to book another oneworld carrier, it might be the best of all worlds!

You may also want to search the relevant forums (AA, Cathay, BA, etc) for terms like "award lap infant" as I know there tends to be more airline specific information there.

Finally, you don't say what kind of travel you're planning, but think carefully before taking a lap infant on any kind of long haul travel...it's a LOOONG trip with a kiddo in your lap, and esp. as they get older, you're going to want your own space.