My policy is that the knife should only come out when it is needed. A careless action (e. g. leaving it on the table) is met with a warning for the first time. If the action is irresponsible (like in your examples) a warning and an explanation is issued. In addition the kid just "volunteered" for some chore (this one was originally initiated by the group).
The second time a kid violates the rules I would keep the knife for the rest of the day.
If it´s really endangering (e. g. throwing the knife) I don´t cut any slack. The knife would be confiscated and returned at the end of the event.
I usually leave the parents out of this. When they care they did their share and if they don´t care it´s in vain. I think that minor misbehaviour should be met with consequences in a short time frame and if doesn´t happen again it´s history. It´s very humiliating for an adolescent when you give her/his knife and a written note to the parents a couple of days after the incident and it isn´t likely to add to the learning experience. I´d rather have a talk to the kid when returning the knife.
Whatever your policy will be it´s important to make it clear and enforce it for every person. It´s equally important that you and the other instructors lead with good example.