Every host is different. If you search for the topic on the community page you will find many many discussions about it. In your profile do you say that you are family friendly? Airbnb recently passed an amendment that says kids under 3 stay free. This caused many hosts to stop allowing kids all together.
We personally do not allow kids under 7. We say that we do not allow kids but then in our rules we say that will not host any children under 7. For the first year we did allow kids but we always charged the extra guest fee no matter how old the child was. Some parents complain but we just explain ourselves and they usually book anyways.
If it were us we would write the guest and ask them to include the child in the guest count. Most likely they will be ok with it. If not you can decide whether you wish to host them still.