Does it matter if your death will moderately increase the happiness of a room full of bunnies? The group-ethics model has some pretty obvious limits, which I think you've violated in your example.
100lb (~45kg) at 50mg is about 900,000 doses of meth. I suspect the death rate per 900,000 doses is quite high and the expected outcome probably includes the death of at least one child.
In my question I'm asking if a simplistic comparative harm analysis would perhaps weigh in favour of non-terminal injury to an infant. It's a very difficult question, of course, and a true analysis couldn't be so simplistic (what proportion of those 900k doses would get replaced by alternate supplies for example). But raising the question seems valid.
Does it matter if the meth might be causing far more pain to more people?
This https://www.google.com/search?q=mother+on+meth+kills+child gives a lot of pretty stark reading material.