This really depends on where you are located and if there are some local laws that forbid you to “forbid” children at your place. Back in November Airbnb wrote their discrimination policies to which we were asked to sign if we want to continue hosting.
In regards to children the discrimination policy says
Airbnb hosts may not impose any different terms or conditions or decline a reservation based on the guest’s age or familial status, where prohibited by law.
This is the reason the system only let you define if your place is suitable for children or not. Airbnb can’t adapt their system to each legislation in the world. If your local laws let you say “no children at my place” then you can add this to your listing’s house rules and your description as well. I know how frustrating is when guests don’t read but that’s all you got.