@Jessica2253 There's really no way to separate it out. When you list a max guest count of 3 people, those searching for a place for 3 will have your ad come up. Many guests don't bother to read through the listing info.
What I would do in your situation is change the max guest count to 2, then say in your ad that you can accept 2 adults and 1 child, that there is a sofa bed that is not long enough for an adult. You could have an extra fee for the child of $10 or $15. You should probably state the length of the bed. There are 12 year olds that are less than 5 ft. tall and 12 year olds that are 5'7".
I would also have a photo of the sofa bed made up as a bed, with wording superimposed on the photo or captioned, that says "Sofa bed is only Xx ft. long. Suitable only for a child."
State in your listing that guests will only be able to enter the max guest count of 2 on their booking, but that they must let you know if they are bringing a child, so you can add the extra fee to cover the extra bedding, towels, and other essentials(or don't charge for the child, it's up to you), and prepare the space for 3, rather than 2.
That's the best I can come up with, maybe other hosts will chime in with other suggestions.