Little kids here are adept at kancho for two reasons. First, they’re just little kids. This means that they’re small, fast, and have no developed sense of empathy or shame. Second, Japanese parents and teachers are remarkably lenient with the under-6 age group. People give the US flak about having spoiled, obese, unruly children but Japan seems to embrace the idea that “kids will be kids” even more. This means that up until they enter elementary school, teachers are at the mercy of the pack. (I should point out that most of the kids are well-behaved, but if you’re perceived as weak or foreign, they attack in groups with the viciousness of the candiru fish.)
For an example of a typical preschool greeting, let the cat and puppies demonstrate.