Back in the LAA/SABRE-days, whenever a flight was not full and a pax had the INF-edit code; the system automatically set an "I"-block for the adjacent seat, in the seat map.
Not sure, if this is still the case nowadays...
Hmm. Now that you mention that, most (probably all) of my AA experience flying with a lap child was on pmUS aircraft before US switched to the AA reservation system, so the experience that I never had seat blocks then may not be relevant to today's AA.