Even my mushroom hating High School Guy ate and enjoyed several beef & mushroom tacos after I made him try a bite. "You're right, mom! That's so weird -- you can't even taste them." So take it from him, and give this a try for yourself!
Baby bella mushrooms will be easier to hide and add a little more meaty umami flavor, but either white or bella mushrooms will do. I used 8 oz of each in my tacos, since that's what I had on hand today -- or you could ease into the meat-and-mushroom idea with 8 oz total, then up the mushroom-to-meat ratio next time if no one notices.