"...so is fingering kids in the ass bad, or not?"
Not if you use a latex glove, you're a doctor, and they've come in for an intestinal problem or you need a stool sample & can't wait until they're ready.
I don't think I could be a doctor. Just like I wouldn't want to be a gynocologist. "Oh really, well happy birthday Mrs. Finklestruben, and you don't look 93 you hardly look a day over 70."