@Janet391
ahh... what were they thinking? Their kids don't need linens and towels ???
The general rule - you are not obligated to let your guest in if more people show then booked.
Airbnb doesn't charge for infants and babies up to 2 y.o. but they should be on the reservation anyway.
You can charge babies as adults if you want and this should be stated in your house rules.
In that case you send them alteration request and just add the baby as the adult or you can request additional money through the resolution center and they should accept it BEFORE check in.
In case your guests show with additional children / guests you can request additional payment through the resolution center and they should accept it BEFORE you let them inside.
(Of course this is in case you already have set additional charge for extra guests over X guests. )