Hey @Kelly395,
I have experienced this problem a number times now and nearly all were Chinese guests. Each time I have called Airbnb and the very first thing I am asked is if I would like to cancel the booking. Airbnb advised each time this was a clear breach of the rules. The first few times I let the couple of extra kids stay but now I don't. The most recent guests absolutely assured me they booked for 4 adults and 2 children. That is not possible with a limit of 4 guests at the property. I am not necessarily suggesting they are being dishonest but I figure if you remember to book flights for your children, and a maxi taxi to carry a full family with children how do you not manage to book accomodation for the children?
I would have a chat with Airbnb so you are comfortable with your decision next time.
P.S. I personally think turning up with 5 undeclared children is taking the p!$s and not acceptable. That said if you can accomodate them you would certainly be entitled to ask for extra money.
Brett