Forever This Summer by Leslie C. Youngblood

Georgie wants to help out at the once famous family diner that served celebrities like the Jackson 5 and the Supremes, but everyone is too busy to show her the ropes and Mama is treating her like a baby, not letting her leave her sight. When she finally gets permission to leave on her own, Georgie makes friends with Markie--a foster kid who'd been under Aunt Elvie's care--who has a limb difference and a huge attitude.