In Maniac Magee, are the "raisins" at the McNab house dried grapes?No

The "raisins" at the McNab house are not dried grapes.  They are roaches.

Maniac is amazed when he sees the inside of the McNab house for the first time.  Never has he come across such squalor; he can smell the living quarters before he actually sees them, and the stench is not pleasant.  In the house,  

"cans and bottles (lie) all over, along with crusts, peelings, cores, scraps, rinds, wrappers - everything you would normally find in a garbage can.  And everywhere there (are) raisins".

The McNabs have "a yellow, short-haired mongrel" who is allowed to relieve itself wherever it wants in the house, and no one cleans up after it.  There is a hole in the ceiling of the dining room, and the peeling paint on the walls comes off "like cornflakes" to the touch.  The kitchen is the worst room of all; a jar of peanut butter lies broken on the floor, and someone has apparently jumped into it and "skied a brown, one-footed track to the stove".  A partially dismembered dead bird lies on the table, and the refrigerator holds only mustard and beer.  There are "raisins" everywhere, but when Maniac notices that several of them are moving, he realizes that they are not raisins at all.  They are roaches (Chapter 35).