Is there by any chance a host setting that allows one to set a basic number of adult guests - say 2 - and then allow an additional 2 guests only if they are children?
The first guest (booking person) must be aged minimal 18 year, an additional guest can be anybody aged 2-12 year (child) or a person aged above 12 (called "adult" by Airbnb).
BTW the listing can be set as "not suitable for children".
@Michael6035 What you could do if you want to be family-friendly is list for 2 guests, but state prominently in the listing write-up that you can accommodate up to 2 children in addition, but the guest needs to discuss this with you pre-booking. And make it very clear that extra adults are not allowed- say it will void your insurance or something and that anyone who disregards this will have their stay terminated.