How does a kid hire a friend without ruining the friendship?

Most of what goes wrong here was never said out loud, so say it.

The problem is that a friend who works for you is two relationships at once and they pull against each other. You have to tell somebody you like that the edges were not done properly. They have to take direction from a peer. Neither of those is natural at thirteen and pretending otherwise is what makes it blow up.

One sentence before the first job prevents most of it. At work I am going to have to tell you what to do sometimes, and that is not me being bossy, that is just the job. Slightly awkward to say and much less awkward than the alternative.

Then agree the terms before anybody starts. What the work is, what it pays, when it pays. Written down. Not because anyone will enforce it, but because a disagreement three weeks later is between two memories otherwise, and memories are convenient.

Agree the exit too, while everybody is still cheerful. What happens if this does not work out? Deciding that in advance means ending the arrangement does not have to mean ending the friendship, and it lets either of you raise it without it being a betrayal.

When the work is not good enough, say something. Short, specific, about the work rather than the person. The edges got missed on that one, can you go back. Saying nothing and quietly fixing it yourself solves today and guarantees next Saturday, and resentment that never gets spoken is what actually ends friendships.

Two more things. Pay on time, every time, even if a customer has not paid you yet. Being slow with a friend's money is the fastest way to turn this sour. And if a customer is unhappy, you apologize and you fix it, not them. They hired you.

Be realistic: some of these do not survive. Learning that at fourteen on a lawn round is a cheap way to find out.