Hi, unfortunately for your request for credentials, this site is a patient-to-patient forum, not a doctor-written forum. This means that although a few people who write in might be doctors or nurses, most are just an informed and interested group of participants with varying levels of knowledge. We'll be glad to give you our opinions, but that is all they are.
As an interested layperson, my comment would be that the science is not settled on any topic surrounding Covid-19, and that (perhaps except the way doctors mask and gown when they are in the ICU or emergency room), there hasn't been a "best" way identified and described to do PPE for people who will not be social distancing.
Given that you are concerned about exposure because of your job as a choir teacher, it seems like you and your wife might want to reconsider having the appointment in person, and instead try a video conference. My husband and I recently had a legal appointment done that way, and though we were passing a lot of data back and forth and the meeting took over an hour, it was surprisingly seamless and not in any way awkward or clunky.
I hope someone who writes in who happens to have medical credentials answers your question.
My understanding having been on this site for many years is that there are no experts on here, just folks like you. Once upon a time there was a separate part of this website that had medical professionals on it, but that's long gone. But here's my two cents worth: people are reporting good results at online counseling. The important face to face is you and your wife, not your and your therapist. I've been in therapy a lot for my anxiety problem over the years, and while getting out and going to an appointment can be helpful if you're anxious or depressed and avoiding a lot of things, it's not as important for actually getting better. That's a crapshoot and is probably more a result of how good the particular therapist is and how hard you and your wife work at it and want it to work. And I've got to say, if you're going to be teaching students in person, anyone who watches the news knows that middle school students aren't complying all that well. Your babysitter is much less likely to be a problem. As for PPE, any mask will do if it's thick enough and worn properly, so you need to test your babysitter by having them demonstrate to you they know how to wear it properly. Finding someone in a family you know is about the best you can do with any babysitter at any time. And your sitter will also have to agree to social distance as much as possible even if wearing a mask, just as your spit-spewing choir students will have to do, as that's about the highest risk activity going. I'm just saying, if you're willing to do that and can figure out how to have people singing around you and not infect you then you can definitely work out the babysitter logistics. You'll be more of an expert than the experts. Peace.