Many (most?) guests have this bizarre belief that small children aren't actual people and therefore don't 'count' when it comes to calculating guest numbers. Of course, this is ridiculous and guests who expect you to accommodate extra people but don't pay for them are stealing from you.
Experienced hosts know that nothing causes more damage and disruption than a small child and I would charge a premium for them if I could, if only to cover the extra cleaning. If you ever decide to allow children, please take this into account and don't fall for that 'children stay free' nonsense. If it has a pulse and isn't attached to a placenta, it pays.
You listing accommodates a maximimum of two. 2 + 1 = 3, therefore the guests choices are (a) someone sleeps in the car or (b) they (not you) cancel. I suggest you communicate these simple choices to them ASAP. These people have decided that they're special and entitled and that your straightforward rules that only two guests can be accommodated and that children aren't welcome somehow don't apply to them. Yes, they'll be penalised when they cancel. This is fair, because it's entirely a problem of their own making.