@Inna22 Put in your listing that the guest count includes adults and infants and cannot go over XX. Then, refer guests to the part of the listing where you state that all guests of any age are counted toward the total. You would absolutely be held legally liable if something happened at one of your Airbnbs and you had knowingly allowed them to exceed the legal count. Also, if you don't charge per person, it shouldn't even be an issue.
Tell the family that legally, your space can only hold XX persons, and so they cannot have YY persons in the unit and that if the arrive with more than the legal number they will be refused entry. Tell them to cancel and give them a refund.