Don't worry. The design is impractical anyway - regulating the "anti-mass" would be extremely difficult. Just when you got it balanced, the kid would drink a glass of water and the board would sink into the ground.
Due caution would indeed be advisable when selling these gadgets, though. A matter-antimatter collision of 50kg would be equivalent to about 2.1 gigatons of TNT, or about 400 times more powerful than the most powerful hydogen bomb ever developed, so it would indeed caused a "considerable amount of damage".