Why could Mr Barnett be so quick to abandon the children?

Mr. Barnett is simply racist. He was looking for an excuse not to serve Cassie and her sibling.

“Mr. Barnett walked to another counter and began filling the order, but before he finished a white woman called, “Mr. Barnett, you waiting on anybody just now?” Mr. Barnett turned around. “Just them,” he said, indicating us with a wave of his hand. “What can I do for you, Miz Emmaline?’ The woman handed him a list twice as long as T.J.’s and the storekeeper, without a word of apology to us, proceeded to fill it.”