"Realistically," it all depends on what legal standards are being applied. I'm unfamiliar with nineteenth century English law and am too lazy to try to look it up.

But given that you say the charge treats the monster as a human child, I think your strongest argument is probably going to be that the monster isn't human and therefore the charge doesn't apply. "It" is a scientific experiment created by Frankenstein, not a natural person (in fact, it's decidedly unnatural). Frankenstein gets off on a technically. He probably would have some liability for his creation, at least in most jurisdictions of the modern US, but the prosecution should have covered their bases and been more careful with the charges.