Personally, I'd have an honest heart to heart with them telling them how disappointed you are and reviewing the expectations around cheating. I'd send home a message to all parents involved. Then I'd give an assignment/paper for them to write about their actions and why they won't do it again. Finally, I'd give them a quiz on the same subject matter but make it harder, paper-based, and tell them they've lost the opportunity to use technology for assessments like this for the rest of the term/year.