Why didn't Mama accept Stacey's Help? What did She mean when she said that Stacey did help her more than he knows?

I'm thinking that you mean the school bus incident in chapter 3?

At TJ's suggestion, they set off down the road rather than wait in the rain, and five minutes later the bus passes, veering close to them and forcing them to jump in the slimy, muddy gully to avoid being hit. A furious Little Man tries to throw mud at the schoolbus from which the white children yell "Nigger!" and "Mud eater!" When Jeremy tries to join them walking to school, Stacey ignores him.

Stacey makes Little Man, Christopher-John, and Cassie promise to meet him in the school toolshed at lunch. They take buckets and shovels and return to the spot where they were forced off the road. There, they dig a ditch across the area that fills with gully water and makes it look as if the road washed out from the rain.

After school, they rush back to the spot that they had worked on. As a result of hard rain during the afternoon, their "yard-wide ditch" has become a "twelve foot lake." They hide in the forest bank and watch as the Jefferson Davis schoobus approaches. Thinking that it is a puddle, the driver drives the bus straight into the hole, breaking the axle and waterlogging the engine. The white kids fall in the muddy ditch on their way out of the bus and must walk home in the rain. The bus driver tells them it will probably be two weeks before the bus can be towed and fixed.