Apple gives my wifi pw away- can this NEVER happen?

No, Apple does not, and has no way to unilaterally give away passwords.

If a password was used by someone, someone else had to share it with them. All Apple does is provide the mechanism to share it, but a person still has to manually want to share it, and actually interact with a prompt to share it. It does not get automatically shared.

Don’t give your kids your password, don’t tap on ‘Share’ when the prompts come up, and it will not be shared.

click here ➡️ How to share your Wi-Fi password from your iPhone, iPad, or iPod touch - Apple Support

Your should NEVER share your iCloud account. Each person needs their own. You can create accounts for your minor children using family sharing.

Apple did not share the password, YOU did.

If the Apple ID is shared and particularly if iCloud Keychain is enabled, then the data will be available to all devices.

That's how iCloud is intended to work.

The security and trust perimeter here is the Apple ID.

When that Apple ID gets shared among different people, you will have issues.

Not just issues with this case, too—with notifications going weird including those for two factor authentication, with continuity sending traffic to unexpected places, etc.

Sharing an Apple ID ends badly, always has, and untangling the usual resulting data aggregation is manual and tedious.

Something else is happening here:

How to share your Wi-Fi password from your iPhone, iPad, or iPod touch - Apple SupportI

Same Apple ID in use, maybe?