Programs that engage young children in movement and song to help them learn are popular but experimental evidence on their impact is sparse. We use an RCT to evaluate the effectiveness of Big Word Club (BWC), a classroom program that uses music and dance videos for 3-5 minutes per day to increase vocabulary. We conducted a field experiment with 818 preschool and kindergarten students in 47 schools in three U.S. states. We find that treated students scored higher on a test of words targeted by the program (0.30 SD) after four months of use and this effect persisted for two months.