Rules get forgotten. Conversations stick.
You can put every control in place and your child will still, at some point, be alone with an AI and a question they would not ask you. What they carry into that moment is not your settings. It is what you have talked about.
Five conversations. None of them take longer than a car ride.
1. "It makes things up, and it sounds certain when it does"
This is the one to have first, at any age.
AI does not know when it is wrong. It produces a confident answer either way, and confidence is exactly what children read as truth.
Try: ask it something you already know the answer to, together, and catch it out. Once they have seen it be wrong, they will never fully trust it again. That is the goal.
2. "It is not your friend, and it does not know you"
AI is built to be agreeable. It will not get bored, will not judge, and will not tell them something they do not want to hear.
That is pleasant, and for a lonely child it can be quietly compelling. Worth naming out loud, especially with teenagers.
Try: "It is very good at sounding like it cares. It does not. That is worth remembering when you are feeling rough."
3. "Assume anything you type could be read by someone"
Not to frighten them, just to set the default. No full names, no address, no school, no photos, nothing about other people they would not say publicly.
Try: the postcard test. Would you write this on a postcard? Then it is fine.
4. "Using it is not cheating. Hiding it is."
This reframe does more work than any ban.
A child who has to explain how they used AI has to understand what they did. A child hiding it has learned only to hide.
Try: "Use it. Just be able to tell me how, and I will ask sometimes."
5. "If it ever says something that feels wrong, tell me. You will not be in trouble."
This is the most important one and the one most families skip.
Children conceal things when they expect the device to be confiscated. If the consequence of telling you is losing access, they will not tell you.
Say the second half of that sentence out loud, and mean it. It is the difference between finding out at the time and finding out much later.
One thing to avoid
Do not make the first conversation a warning. If AI arrives in your house framed entirely as a danger, your child will use it anyway and simply stop mentioning it.
Curiosity first. Caution woven through. That order matters.