@Mani-and-KP0
If they book for 2 people and in a message thread they say they will come with a child (or they just show up with a child ) then you can change their reservation and include a child. They have to accept it. Do not let them inside in your property and check in before they accept it. If they don't want to accept then contact Airbnb and ask them to cancel this reservation.
If you want to charge for infants up to 2 y.o. than the process is the same but instead of an infant you just include a child.
but in that case you should have it in your house rules that you charge every guest including infants.