In Maniac Magee, at the McNabs' house, Maniac thought that something was raisins. What was it really?
The answer to this question can be found in Chapter 35 of Maniac Magee. In this chapter, Maniac has gone to live with the McNabs. He has met the twins and has decided to go live with them. When he gets to their house, he finds that it is a terribly disgusting place. There is a dog that is urinating on the floor and no one cleans up after it. There are apparently feces from the dog in the corner. There is a huge hole in the ceiling of the dining room. But none of that is as bad as what is in the kitchen.
It is in the kitchen that Maniac sees the things that he thinks are raisins. He notices that there are many of them all over the place. On p. 132, the narrator tells us
The raisins here were even more abundant. He spotted several of them moving. They weren’t raisins; they were roaches.
All of this is meant to show the squalor in which the McNabs (whose older brother is a white supremacist gang leader) live.