When I first started hosting I was too intimidated to ask to be paid for an additional guest who showed up unannounced. Now I tell them it's fine, and remind them there's a $30 per night charge. I've never had a problem collecting.
As for children, this is a problem for me. My listing says not suitable for children, but Airbnb says we have to take infants (0-2 years) without charge. This is unfair. I have found guests with babies/toddlers use more of everything. Water, towels, soap, shampoo, electricity, etc.. and they are usually here a lot more than adult guests. I just got a request from a man showed only one guest, but says it's for him, his wife and their toddler daughter. It's for Christmas week, and he asked for a discount! I asked him how old his daughter is, and if she's older than two I don't think he'll be staying, because he won't want to pay the extra $30 per night.
Since Airbnb says we can't charge for infants, does that mean we have to accept them at all? My place really isn't suitable for children, especially toddlers.