I'm pretty sure you don't ride pararell to each other on dh tracks. Also if the kid gets out of control on a dh track he will just land on a tree so unless Treebeard decides to sue the resort what's the problem?When they start making DH tracks 100 meters wide, then it will be OK to have 6-year-olds on them.
@Alexis-dh the same may happen to any slow rider.
Kids should be allowed to ride as early as possible if we want our sport to progress. I remember reading an article on guardian about how one of the main reasons of records getting broken all the time is people getting into the sport at a younger age (of course more people helps too) but if you really want snowboard comparisons - Shaun White.