Our son outgrew the infant-in-arms stage a couple years back, but we flew many times with him as an IIA in all classes of service. The only thing I have to add to the correct information posted above is to not delay taking care of this. IME it is 10% of the prevailing ticket price at the time you add IIA, which may be much higher than the ticket price you paid. This was certainly the case when we cleared into D1 at the gate on a GUC from HKG-SEA; the fee was quite high in that instance but we were more than happy to pay it!