Gamma Quick Kids 36 Tennis Balls - 12 Pack For young players starting the game and learning the art of spin, the GAMMA Quick Kids 36 Tennis Balls feature a two-tone felt for easy instruction. The Quick Kids 36 balls are USTA approved for 10 and under tennis and ideal for ages 8 and under on a 36' court. These lightweight balls feature a low compression core making them 75% slower than a standard tennis ball and providing a lower bounce. USTA/ITF Stage 3 Specification.