If your site uses Guesty, you cannot set up an offer based on the number of children booked, such as one child free when three children are booked. Offers of this type are not available.
Why this happens. Guesty prices a stay on the total number of guests, and your website sends Guesty that single guest count. There is no place for a rule based specifically on the number of children.
What you can do. Check with Guesty support whether their promotions can be conditioned on guest count.