Hey there! Hello!
I believe the comma should go here:
"I saw three kids sitting on the porch, across the street."
"Across the street" is a sentence fragment. These are typically separated from the rest of the sentence with a comma. Sentence fragments may seem like sentences, but you couldn't make it it's own sentence, since it doesn't contain a verb. A sentence must have a verb and a subject in order to be considered valid, and in the case of fragments, they're purely there to add more to the current sentence.
I hope this helped you out! Feel free to ask me any additional questions if you have any. :-)