The judge was already no spring chicken

When Sweeney Todd was transported to Australia. Really, the simplest solution was for Joanna to poison him, make it look like a natural death of old age. Or the result of senility, perhaps - he’s plausibly senile as it is! She seemed to think it’d be easy enough to poison herself, after all. A little ingenuity and she would have been set for life.

Well, I'll admit, this is stumping me

You'd think a middle grade portal fantasy with multiple color illustrations - but not a graphic novel! - would be easy to find.