The students were not supposed to do magic over the summer, full stop. There's no official exception there. The leniency could be rationalized as "magical parents can stop their children from casting spells if need be, so we don't need to monitor them," but it's not "go ahead and do magic, magical parents are trusted to teach and guide their children's magic."

If the children are casting spells, then they are breaking the law. If the parent is teaching them without the child actually casting the spell, then there's no need for an exemption.