When i try add game and/or app for kid account, after enter password confirmation on my...

From what I remember when seting up a child account an adult has to enter a password (I think it may be linked to an live id). When upgradeing a child account the password will have to be used to confirm the account holder is now an adult and not just some
kid trying to bypass the child account his parents put on his xbox.

Try refunding it from their microsoft account online. Worst case scenario use that money from their account to gift games to your main account. You main account will own it. Also, you can put a password in so that in order to confirm a purchase you have
to enter a password so they wont "accidentally" buy something as i probably would if i was a kid, lol.

try this.

When you are prompted to enter your passkey on the console, enter any six-digit combination three times.

After you enter a passkey three times, Xbox One will prompt you to enter your Microsoft account password. Type your password.

Select Make a new passkey.

Enter a new passkey, and then enter it again to confirm.

if you don't know your password you'll have to reset that as well.

My kid did this back last year, there is an option in the settings so that you have to enter your Microsoft password when making purchases. Unfortunately my kid is too smart for a passcode and worked out the pattern from watching me lol and even now I
have to use my xbox app to enter passwords as he can see me entering password on screen 🙈.... The joys of them learning to read 🤣

Could you try these steps below.

1. Press the Xbox button to open the guide.

2. Select Settings.

3. Select All Settings.

4. Under Account, select Remove accounts.

5. Select the account that you want to remove.

6. Select Remove to confirm.

7. When you're finished, select Close.

8. Restart your console.

9. Press the Xbox button to open the guide.

10. On the Sign in tab, move all the way down and select Add & manage.

11. Enter the email address for the Microsoft account, and then select Enter.

12. Enter the password for your Microsoft account, and then select Enter.

If that doesn't work, could you try a factory reset and selecting keep games and apps when prompt to.