@Rachel294
This is a common problem. Some people have decided that infants don’t count as guests.
You should make it as clear as possible in house rules that EVERY human of ANY age is considered to be a guest and the total number of guests allowed is FOUR, NO EXCEPTIONS.
For people who have already booked, you can inform them that no more than four people will be admitted to the premises. If they show up with more than four the extra ones will have to stay somewhere else.