Hmmm cany help feeling that this all a bit of an academic discussion.
Fundamentally the safest way to transport infants is in a proper infant seat and not on an adults lap.
Briefing an adult with an infant during a planned emergency might take more time, but so would any blind or deaf adults, or foreign language only passengers - so should their number also be used to calculate the required number of flight attendants? Where do you draw the line? Safety in the real world sometimes has to take second place to commercial pressure.
Maybe the best thing would be to ensure that the SLF actually pay attention to the pre-flight safety demo properly. Mind you the configuration of some cabins and the number of demos carried out means that some pax can't always see the demo anyway.