Need help on sentence punctuation, please!

I'm no grammar expert, but the comma is fine. As would be breaking it up into two sentences, with a period where the comma is now.

annsquared said:

Abandoning him isn't an option, not after he gave up his childhood to keep them alive on the demon-infested streets of Detroit.

Comma or semi-colon after option? Or just throw an em-dash in there and call it a day?

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The comma is perfect. A dash will also work. You can't use a semicolon after option, because a main clause doesn't follow. Semicolons need main (independent) clauses on either side.

For that reason, a period after option with leave a fragment. Fragments are okay in novels, but they're more effective when shorter.

annsquared said:

Abandoning him isn't an option, not after he gave up his childhood to keep them alive on the demon-infested streets of Detroit.

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The punctuation is correct as is, but you might consider a change in verb tense for one phrase. Two options:

1) Abandoning him isn't an option, not after he had given up his childhood to keep them alive on the demon-infested streets of Detroit.

2) Abandoning him isn't an option, not after he giving up his childhood to keep them alive on the demon-infested streets of Detroit.

Using "not after he gave up" implies abandoning him wasn't an option right at that moment after he gave up his childhood, but abandoning him might be an option later.

But using "not after he had given up" or "not after giving up" implies abandoning him wasn't an option from that point onward, which is what I think you mean.

You can't use a semi there, as Chase said.

More problematic is what seems like the unclear antecedent you've got going on though - unless the abandoning-or-not one is female.