You start getting fined at 15 and up, so it would make sense to get things organized on an ED visa before that. Under 15 I would imagine a lot of kids go to school with no visa and have no problems. Don't have personal experience on the subject however.
Children under the age of 15 are not charged a fine for overstaying but still get overstay stamps when they leave the country. Not 100% sure of any future issues, but probably none.