Yeah, this.
After some thinking on this, I'm taking back my initial instinct to allow command to break the Concentration with a word of "sleep" (or most anything else, for that matter.) The reason is that the broad tactical "purpose" of command is to deprive the target of their actions. Allowing much more than that makes it too powerful for a 1st level.
With that said, I still think "sleep" is a valid possibility for command, but the effect is something along the lines of the "grovel" option: the target gets drowsy, maybe lies down, and starts trying to snooze. But that's all.
If a caster wants to break a target's concentration, just inflict some damage or incapacitate it "for real," eg. with sleep. Or a maul.