Mystery at the Ballpark (Boxcar Children Special Edition #4) by Gertrude Chandler Warner

Jessie and Violet join Greenfield's new baseball team, and Henry and Benny get jobs helping out. But someone is rooting against them. A special bat is stolen, Jessie's glove disappears, and the team almost misses their first game.

Can the Aldens solve the mystery and save their team?