The word הכל allows omission of the את, though doesn't require it, so both versions work.
"They shave it all" would be more accurately translated to "הן מגלחות את כולו/כולה", to account for the pronoun. But it might be acceptable here. Report and have the moderators decide.
As Naftali wrote, both הכל and את הכל work here.
Not sure what you meant by את הכל ברוש. Did you mean by any chance "the head"? Then, it would be הראש, not ברוש. But there is another thing - if you want to say "whole head", you'd say את כל הראש. You can't have anything after את הכל, unless it's a subordinate clause.
Well, I thought Duolingo-Troll wanted to illustrate a point, Lewert Glinert (§5.1) makes too: I eat anything, i.e. I am not a picky eater, but אֲנִי אוֹכֵל אֶת הַכֹּל I eat everything, i.e. all that is there on the table. The אֶת makes the statement more specific. PS. רוֹשׁ is hemlock.
Sorry guys. Yeah, meant ראש. Just trying to say "We shave everything on the head." (As in every hair on the head or every place on the head where hair grows)
Btw, Ingeborg, you did a favor for me I could not respond to a few days ago. Thanks for giving it a shot. The term "fellow creatures" has long been problematic for me as well. I did throw a lingot your way.