Do I need new account for second Xbox

You would need another account. They can't be signed into the same account on both consoles at the same time to game so it could cause issues if they were to try using the same account. It's also better for the future if they don't share accounts. How to create an account

As far as the subscription, they can share that. If the account they are currently using has the subscription, have that account set one of the consoles as its Home console. Then that account would use the non-Home console and the other account would use the Home console to access the subscription. https://support.xbox.com/en-US/help/hardware-network/console/my-home-xbox

Yes you can use your account on the second console you have. Just sign in with your account details and all your digital games will be available to be installed on the console.

For a second account you will need a new Microsoft Account (new email). Like was said above you can do account share to access all games and subscriptions from the another account, but to login with a different user you will need always a new microsoft
account.

I have done this suggestion (for the second time actually) my old adult account is now added to my new family account as a child account and i have set the time limits. i have done all this via my pc but the xbox is not stopping! do i need to do something
via the xbox?

No, they do not have an account

That is the issue, you need to create a xbox account for the second player, even being a splitscreen is required a second account.