Should my kid go inside a customer's house?

For some work, yes, and the distinction that matters is not inside versus outside. It is known versus unknown.

A family from church, school, or three doors down is a different situation from somebody who responded to a flyer. The first is ordinary. The second needs you involved before anything is agreed, and that is not overprotective, it is what any sensible adult would do for themselves.

Plenty of good work happens indoors. Babysitting, pet care while somebody travels, tech help for an older neighbor, tutoring. Ruling all of it out removes some of the best businesses available to a kid, particularly a quieter one.

What to have in place for any indoor job:

You know the address and roughly when it starts and ends. Your kid has a way to reach you. And for a first job with a family you do not know, you meet them first. Nobody reasonable finds that strange, and anyone who does has told you something useful.

Door knocking is the firm exception. When your kid is going door to door, they do not go inside, ever, for any reason. Not to see the job, not for a glass of water. That one should be an absolute rather than a judgment call, because the whole point of an absolute is that it works when a kid is caught off guard by somebody being friendly.

Then the rule that outranks the rest. If they feel uncomfortable, they leave. Immediately, without being sure, without explaining, without worrying about the money or seeming rude. Say it plainly, because a kid who thinks abandoning a job means being in trouble will talk themselves into staying.

Worth a conversation up front for tech help specifically: the work happens in a shared room, not a bedroom, and older neighbors are almost always fine with that.