Fist fighting in middle grade novels

Nope.

People have a very strange idea of what is too much for children. Many MG's have way more violence than that in it. Have you ever read Peter Pan? Let me quote you a section:

"Let us now kill a pirate, to show Hook's method. Skylights will do. As they pass, Skylights lurches clumsily against him, ruffling his lace collar; the hook shoots forth, there is a tearing sound and one screech, then the boy is kicked aside, and the pirates pass on. He has not even taken the cigars from his mouth."

So yeah . . . don't worry about it . . .

Probably the more fantastical the story, the more violence you can get away with. MG characters kill bad guys all the time, as long as they're monsters or in some other way not quite "real."

In a contemporary, realistic novel, I think if it fits the story, it's probably fine if it's not graphic or bloody. I mean, fist-fighting? What planet was your critiquer on, to think young kids never punch each other? (Oops, that sounded snarky, sorry. I'm sure your critter is a fine, wise, noble person. which may be the problem.)

As a teacher, I can tell you: Kids hit each other a whole lot more than they should. Sometimes for absolutely no reason. And in my district, you can be suspended for fighting something like 20 times before you get expelled.

If your book reflects reality, it should be fine.