Ok, I'm not a freedive instructor, but I've helped a few beginners. Also at the moment I'm working on my professional swim instructor ship, I teach children how to swim, young ones too! 4-8 year olds typically.
For their diploma's they also need to swim 3 - 6 or 9m under water, here my years of Freediving really pay off

LOL. My guidance consist of technical direction of diving deep, and swimming slow with big strokes. Also to exhale deep and inhale before diving. Swimming along the bottom is easier too. And being relaxed and quiet before helps too.
Now if they are into underwater swimming I would teach them how to snorkel, duck-diving, taking snorkel out, clearing mask and snorkel, finning and buddy system and response ability.
About breath-hold I would explain about the basic relaxation = nicer diving. Do some relaxation practice, Watsu stuff. Also I would say as preparation, relax, easy slow breathing, exhale, slow inhale and dive. When to 'come up' is, when your urge to breath starts, then slowly and relaxed come up. So if you relax well you get rewarded with a pleasant en longer dive.
But then again it's all a dangerous endeavor, since children are very often not mentally conscious, they are not able to think through things - see and predict danger, and are constantly challenging each other and themselves to push boundaries.
An example is a friend of mine who described to me how far he pushed his dynamic when in junior school. He said his view went black in the end and was disorientated for a few moments pushing to 25m.
At this moment I believe in explaining dangers in simple steps with the young, putting understanding and response-ablitity in their hands, the difficulty is that they are not my children and ultimately their parents decide.
However these parents should understand that if they are not around their children will still experiment, challenge and push themselves exploring boundaries.
As an instructor you face a dilemma, granting knowledge will help people to be able to dive saver, but it also enables them to push a bit further. The safety then lies in the value the individual sees in (his/her) life. Forbidding a hobby or passion will put a dent into a persons appreciation of life.
The danger is when a child suffers an incident or (God forbid) death in water, you can get blamed.
Now doing dangerous, challenging 'individual' stuff can be very rewarding, as I'm sure many here have enjoyed. The mental
inner dialog in search for limits and improvements surely help a person become an aware, balanced and knowledgeable person. Someone who also can recognize the psychology in himself and others making him very likely to be suited for a management position in life. Learning to manage yourself is the first step. (I'm still working on it btw.) Maybe you find it interesting to know that elite universities do not look much at your high school grades, but what kind of things you do with your free time. In this they look for 3 developmental categories: Art, social, individual.
This is a video discussing Dangerous Things in general, including fast cars and guns.
Sorry for the disorganized rand, I'm just to tired and lazy to perfect the piece.
Hopefully it's of use, and I love to hear and learn from other people's views.
Love, Courage and Water,
Kars