I’m finished (with) trick-or-treating.

[diamondcutter]

“Where are you guys heading?” he asked us. “I’m finished trick-or-treating.”

Source: Mrs. Patty Is Batty, a children’s novel by Dan Gutman

I’d like to know if the preposition “with” is omitted after the word “finished” in the last sentence.

 

[emsr2d2]

It's not omitted; it's optional. However, adding "with" changes the meaning (for me, at least).

I'm finished trick-or-treating = My evening of trick-or-treating is over. Now I'm going to do something else.
I'm finished with trick-or-treating = I've had enough. I'm never doing it again!

Note that BrE speakers would use "I've finished" in the first sentence.