Your mileage will vary, but the way I see it, you really can’t lose here, as long as you can get them to follow the same testing procedure as everyone. If you let this kiddo do percussion, you’ll either have a kid that works out to be a decent or better percussionist, or you were right and they should have played something else.

Still have the student tryout following your process, mine is every student tries every instrument but I take personal preference into consideration, have the parent present and let them see what they do well at. Explain that everyone has to follow the process. But if they’re that adamant about it, the kid will “magically” be best suited for percussion. I guess this ultimately a case by case basis, but the process is the same for all.

As an antidote, I have a friend at another school who once had a beginner test best for tuba. Parent was pushy for saxophone, they compromised on clarinet. That kid ended up being a 4 year all-stater.