These may be two little boys, but they're about as different from Hester and Lester Beale as you can get. Hester and Lester are naturally loving, but Russell and Piper are naturally ready to fight. And then steal your wallet. And yes, they do know that stealing is wrong: "The other smacked him with his cap. 'Shut up Piper, you stupid sausage. You don't go telling people you stold stuff.'"
But it's not all their fault: there hasn't been much love at the McNab household. And by the end of the book, thanks to Maniac's care, we start to think that there might be hope for these kids after all.
You have to admire their ambition. The McNab kids don't like school, and they're not happy at home (shock), so they take off for Mexico. Yes, it concerns us a little that they likely have absolutely no idea where Mexico is or how far they have to go (there's more than a few states between Pennsylvania and the Mexican border).
But the important part of their ill-conceived plan is that they're going together. Notice that Russell and Piper are basically inseparable? This is true for much of the time we know them. Maybe this is what actually gives them a chance. Unlike their big bro who has really had to find his own way, Russell and Piper have built-in support.
They've also got Maniac. Maniac comes along for Russell and Piper at just the right time. What would have happened if he hadn't stopped them from going to Mexico? Who would be making personal sacrifices to make sure they aren't six-year old dropouts? Maniac may not remember his real family, but he already understand how families act:
As for Maniac, he understood early on that he was being used for the greater glory of Piper and Russell. He also understood that without him, they would not be going to school every day. For the McNabs, there was nothing free about public education. A tuition had to be paid. Every week Maniac paid it. (37.9)
With the little McNab boys, we see total breakdown. There's nothing innately good about families: family love, like school, has to be bought and paid for with care and attention. Luckily for Russell and Piper, someone cares enough to fork over the price.