In what chapter were the twins, Piper and Russell, introduced in Maniac Magee?

The twins Russell and Piper appear in Chapter 34 when they are trying to run away to Mexico.

Maniac is at Valley Forge, where he is planning to live in a recreated cabin for a while, when he realizes that there are a couple of kids there too.  They had the same idea.  Maniac goes to check in on them, nicknaming them Missing Tooth and Screecher in his head. 

Maniac asks them where they are...

The twins Russell and Piper appear in Chapter 34 when they are trying to run away to Mexico.

Maniac is at Valley Forge, where he is planning to live in a recreated cabin for a while, when he realizes that there are a couple of kids there too.  They had the same idea.  Maniac goes to check in on them, nicknaming them Missing Tooth and Screecher in his head. 

Maniac asks them where they are going.

The answer came from both: "Mexico!" Maniac bit back a grin. When they stood, he saw they couldn't have been more than four feet tall or eight years old. "Well," he said, "it's good and warm down there, but it's pretty far, you know." (Ch. 34)

The kids have a stash of junk food with them that they clearly did not pay for.  Russell scolds Piper for telling Maniac that they stole it, but Piper assures Russell that it’s okay because they did not tell him where they stole it from.

The boys are afraid Maniac might be a cop.  He has the pizza from untying the knot, so he tells them that he’s no cop--he's a pizza delivery boy.

"Not me," said Maniac. He moved in from the doorway. "I'm" -- and with only a moment's pause, the story came to him -- "a pizza delivery boy. We have a contest every week, and you two were chosen for a free pizza." (Ch. 34)

The boys buy the story that Maniac came to deliver them a free pizza.  Although he is a runaway himself, Maniac convinces the boys to return home.  They can go to Mexico the next day.  He promises to show them a shortcut.

Maniac returns the boys to their big brother, "Big John McNab" of the Cobras.  Of course, he is wondering what Maniac is doing with his little brothers!  The kids are impressed when they find out that the famous Maniac Magee delivered them a pizza.