Don't cancel the booking. She's the one who's acted dishonestly and she should be penalised, and you rewarded. Message her back and say "As it states in my listing, the maximum number of guests I can accommodate is two. Whilst we are keen to honour your booking for two guests, we are not able to accommodate six. Unfortunately only two guests will be able to stay overnight. Unless you are able to find alternate arrangements for the other 4 members of your party, I strongly advise you to cancel. Can I also suggest you take the time to review the Airbnb Travelling with Children guidelines, and follow them. These state "If a host adds Family/Kid Friendly as an amenity, this implies that infants, children, and families are welcome in their listing. You'll still need to disclose the total number of guests, including infants and children, accurately when making a reservation request."
Also, if you are prepared to accept more guests, then you need to change your policy to charge for every single guest, regardless of age. As most experienced hosts will attest, nothing is more likely to lead to damage than a party that includes children. Also, families with children, particularly infants and toddlers, use far more resources and are more costly to host than any other demographic. Most of us would charge a premium for hosting the under 8's if we could 😞