It doesn't sound problematic enough to stop a potential editor from buying the book. That said, it really boils down to whether the scene is necessary. Is this part of a larger issue? What does it offer to the story as a whole? Is this issue integral to the plot?
In my first MG, the editor asked for a sentence to be removed bc of the abuse content in it. She was right to do so. It was established that the character had an unhealthy home life, so that extra line was unnecessary. (Note: The line was FAR less violent than the example you cite.)
OTOH, there were a couple sort of double-meaning typical 12 yr old boy jokes that I thought might be a shade too far, & the editor was fine with those. I even had her check with the School & Library team who also said I was worrying too much. So, I think that when you go into the editorial acquisition & revision process, there is always the potential for surprises on what is/is not a cause for stopping & considering.
RE: Your comment that
there are other parts of the book with fighting, but they don't concern me, because it's "fantasy." Only the part with the "real" family concerns me.
I think you are right to think this way. My MG series is fantasy. There are fights with trolls, wolves, draugr,and various mythology beings. None of those were an issue. It was only the familial situations that were. We seem to have different standards for human-on-human, human v monster, & human v family.