You say it "may not be" suitable for children. Since you have instant book on they don't have to message you first to discuss this, and that gives the guest leeway to decide it is suitable for THEIR child. You could be clearer and say "no children under 12." If I were you, I would charge for the third person.
One of my houses is not child-proofed and it would be a safety issue to allow children to stay there. I don't want to violate the commitment, so can you tell me where I could read this?
@Ann72 Airbnb changed it from "Not Suitable" to "May not be suitable" a while back. Legal compliance maybe?
@Noeline1 I noticed there are now fields for Childen 2-12 and under 2.
Don't cancel, Call Airbnb, the guests were not forthcoming about who was coming and thus didn't meet your requirements so maybe you will get a pass. Good luck!