Hello Ciara
If you can (BnB has made it mandatory for some new hosts), I would suggest you turn off instant book. This will allow you to talk to guests through the messaging system to make sure they are a good fit, before confirming their bookings.
All the more important when you have young children.
It's unfair to cancel a guest's booking unless absolutely essential as they are likely to have made travel and other arrangements around this. BnB takes this very seriously - will block the dates out in your calendar, stop you from having Superhost status for a year and it will be highlighted in your listing, which will put others off from booking.
I would give BnB a call and explain that as a new host you didn't understand the system and see if they can be flexible on this occassion.
Do spend some time to familarise yourself with how BnB works - you are running a business by letting out accommodation and it will really help you if you are familiar with how BnB works.