Boy! Do something weird, no matter how small and I'll lock you in your cupboard until Christmas.
Firstly, is してみろ actually imperative here? I don't see what else it could be. Secondly, I'm assuming this is a natural sentence (which past experience with the Harry Potter translation has shown to not always be the case).
This sentence seems somewhat analogous to "try something funny, and I'll give some punishment" where, in isolation, "try something funny" looks like it's an imperative, but really it's just omitting an "if you" at the beginning. Obviously the exact same thing isn't happening in the Japanese text, but I wonder if there's some way to think about this sentence* that makes it clearer what's going on?
*I say sentence, but really there are two here. The first 。is not a typo and the fact that the sentence ends here is also a puzzle to me.