It's not possible to check the current password, but the password for a child account can be changed under Settings in the parent account.
You can change the child account password and then sign in to the account using the new password. Please follow the following steps to change the password.
How to change a child account password
1. Go to the Nintendo Account sign in screen using a computer or smartphone and sign in to your parent or guardian account.
2. Select "Family Group".
3. The members of your family group will be shown. Select the child account for which you'd like to change the password.
If no child accounts are listed, it's possible the account you're looking for is linked to a different family group (under a different Nintendo Account). Please sign out and try logging into any other Nintendo Accounts to which the child account might be linked.
4. Select "Sign-In and Security Settings".
5. Select "Change Password".
You can also confirm or change your sign-in ID here.
6. Enter the password for your parent/guardian account.
7. Enter a new password for the child account.
If an error message is displayed, click cancel and try changing the password again. If the error persists, please sign out and back in again. You may also need to update your device or browser.
If you are still unable to change the password, please contact us for additional assistance.