Next time give them dishes,glasses,ashtray's or vases...
From experience I know they can throw further and more accurate then
But then they also need a good incentive lol like being really angry at you. xD
Mythbusters tested this in one of their episodes and found out that it's not true. Boys are usually better because they are trained from a young age and girls aren't, but when using their left hands, both were equally good.