If you are a parent and want to sign in for more than one person (e.g. yourself and a child, two children, etc.) - make sure all the accounts have the same email address and password. You might need to sign in to each one to begin with and make this change.
Then when you sign in you will be given a choice as to which account you want to sign in with. This saves you having to remember multiple passwords. However, you can only sign in with one account at a time.