What are all of the boys bringing into the dining room of the McNab house in Maniac Magee by Jerry Spinelli?

The boys are bringing cinder blocks into the dining room.
The McNab house is pretty unusual.  Manaic Magee stays there because he wants to help Piper and Russel, but he finds the house messy and disorganized.  The Cobras do not share the same values as Manaic.  He does not see race, and they are a very racist group.
The McNab house is hardly clean.  Still, it can get worse.  Maniac doesn’t approve of the cinder block wall they are building for their revolt against the African-American East Enders.  He still finds it hard to believe they finally started.

The living room was hazy with dust. At the back end of the dining room, they were bringing in the cinder blocks – George and John and a handful of Cobras -- lugging and grunting them in from the backyard and dumping them onto the floor. (Ch. 39) 

Maniac is a very level-headed kid.  He wants everyone to get along.  When he first came to town, he didn’t even understand how racially divided it was.  The McNabs are a perfect example of racial tension at its highest. 

Once it was done, they'd be ready. Let the revolt begin. Let the "rebels," as they called the East Enders, come. Let'em bust through the newly installed bars over the plywood on the windows. Let 'em bust through the steel door. They'll find themselves staring down the barrel of a little surprise. (Ch. 39) 

They can’t decide what kind of big weapon the surprise will be, which is just as well, since they don’t actually have anything.  Clearly, these people are not willing to be open-minded and learn how to see eye to eye with people of other races.  To them, the blacks on the East End are the enemy.  That is how it is, and that is how it always will be.
With Maniac and the McNabs, the author presents two very different attitudes toward race.  In many things Jeffrey "Maniac" Magee is naive, but race is probably the biggest area.  He wants everyone to get along.  He thinks that race makes things interesting, and having people of different colors makes for better neighborhoods.  This is a very unusual perspective, but does demonstrate how racism is learned and taught, and how there is a chance with each generation for change to be possible.