I recently had a similar situation, but with a reservation through VRBO. The guest is an Asian gentleman whose first languagge is not English and it was clear to me that his grasp on English was not great. He booked for 8 people, which is the max my place will sleep. I sent him a messgae asking for the names and ages of everyone coming. He waited a week before he sent me a list of 10 people, 4 of which are children between the ages of 8 and 11. I immediately called him and explained my place only sleeps 8 with 4 beds, and that 10 is too many. There will be nowhere for 2 people to sleep. He thought the children didn't count or matter in the count, though he did originally count for them. My goodness! Where did he think they were going to sleep? He originally booked it for 8 but then his parents wanted to come on the trip, so by the time he sent me the list a week later, the number was at 10. He went to cancel, but it was too late by a few hours for the cancelation policy. If he canceled, he would lose half his money as he paid for the whole thing up front. (This is why you respond to your messages right away, people!) Anyway, the long story short is that my husband and I didn't want him to lose money so we agreed to provide a mattress on the floor in one of the bedrooms for the children. We will take it from our own hide-a bed, since that really is all we have. The guest seemed happy with this solution and even offered to bring sleeping bags, to which we said no because we do have extra bedding for change overs. I also took some of my picnic dishes and put them in the apartment to supplement the dishes, since there is only a service for 8. We agreed over the phone that he will pay us the $24 extra for the 2 people over limit ($6 each for 2 nights) in cash once he gets here. The money really won't cover the extra effort but we'll bite it. It's Christmas time. I only hope that in the end, we get a decent review from this.
Since then, I've put in my listings VERY CLEARLY the maximum capacity of people my place can hold, regardless of age, and that children count as numbers toward that. I doubt having it there would have helped with this gentleman due to the factors mentioned above, but it certainly can help to weed them out later on and will be good backing for a justifiable cancelation down the road, should it become necessary.
AirBnB may not charge for children under 2, but that doesn't mean that you have to accept over capacity numbers in your place. Just be very clear in your listings, like I have now. What you do now will be entirely up to you how you want to solve it. If you do require an extra charge, be sure to go through AirBnB's request form through the site. I didn't do that because this booking was not from AirBnB and it would have been way to complicated to explaint to him how to do it.
*Also, it's entirely possible that they have triplets who are 18 months old. It's not as uncommon as it use to be.
