I've got to say, I'm struggling to work out what the problem actually is.
Are you unhappy that sometimes she brings another person to your house? Fair enough, your house, your rules, I don't always like people tagging along when I have work done at home - it all depends on what I'm having done and who the person is, etc.
Or are you unhappy because the other person doesn't clean as well as your cleaner? Again, fair enough, and that's your right.
But what you actually said is "I’m not completely comfortable with paying a 15 year old the same as I pay his mum", and yet you don't tell us why you feel that way. Do you know why you feel like this? If so, let us know. At the end of the day, he may well do the job better than someone much older, but without knowing what your rationale is for how you feel, it's impossible to answer.
However, the fact remains that it's your home, and you are well within your rights to ask her not to bring her son with her. You don't need to give a reason, though she may well ask you why - and that's why you need to be true to yourself first, because if you don't know, then you can't give her a logical answer.