One Crazy Summer by Rita Williams-Garcia

In One Crazy Summer, eleven-year-old Delphine is like a mother to her two younger sisters, Vonetta and Fern. She's had to be, ever since their mother, Cecile, left them seven years ago for a radical new life in California. But when the sisters arrive from Brooklyn to spend the summer with their mother, Cecile is nothing like they imagined.
While the girls hope to go to Disneyland and meet Tinker Bell, their mother sends them to a day camp run by the Black Panthers. Unexpectedly, Delphine, Vonetta, and Fern learn much about their family, their country, and themselves during one truly crazy summer.