This is a very tricky one. The example of PH vendome not allowing 5 mo old infant is just nuts.
Many hotels including some US but will show no availability if you put 4 guests - even though two of those guests are infants. It gets worse - even many larger rooms/suites often have max occupancy 2 or 3 loaded so if you enter 4 you'll get nothing.
This is even more complicated when you book award rooms.

We always put 2 and email the property directly right away asking for cribs. As we almost always book suites (or two rooms) we never had a problem but a couple times it required follow up emails.