Before I start my rant, let me define what I call a one-shot. To me a one-shot is a self-contained adventure in which is intended to last one session or less. You may not share that definition, but all future references to "one-shot" in this post are to be defined as that.
I've found that Grimm one-shots cannot be treated like say, DnD one-shots. I find Grimm to be especially story-driven and revolving around a central theme. Say if you stick to the book and are having the kids attempt to escape Grimm, that should be the driving force of that chapter. Hence it will take some time to get your kids into Grimm, introduce the world, etc. This is in stark contrast to a genera d20 game where you can plop your adventurers into a tavern, tell them the evil wizard is over there, and they accept it without pause.
Part of what I discovered is that L6 is just ...different from d20. What a lot of my players will do is run skill checks as if this were DnD, they tend not to have a lot of fun. In fact, they find the game boring because generally whatever they want to happen happens. So they climb the tree...or seem cool to the peasants...etc. Needless to say there was never anything deep happening in the game, because I had just plopped the party down somewhere and let them do whatever they pleased. BIG MISTAKE! IMO, Grimm cannot function properly without direction and plot. Those 2 factors will be much more important than overall length. You can run a quixotic campaign if necessary, a chapter here, a chapter there, graduate your character 4 times, play again, etc., if you want and I see Grimm functioning fine. You can also run a full-fledged campaign and it would work as well. That to me is a question of personal preference. However, if you choose to do a one-shot, take the time to develop the plot and direction. Random dungeons with random monsters is not what Grimm is all about.