While a very good question, one has to remember that children generally do not join atheist organizations. Unlike religious movements, rarely are all families involved and then rarely do these groups consist of regular and repeated situations where children are left to fend for themselves for hours at a time.
Religious groups generally have activities and schools designed to teach children and get them involved. Religious practice is generally taught from infancy in some of these cases, and religious groups have clergy who may be entrusted with children for varying lengths of time without parental supervision. Unless a non-religious organization does the same things this would be a classic "apples and oranges" situation that would prove little.
Instead we need to find if any non-religious organizations have similar patterns and then compare those that do, and then we would have data worth something.