You need to create a new username. You can only file your own return in the account you are in. You cannot file another return, even for a dependent.
Sign out of your own account and start a Free return with the link below. Ensure that you are signed out and your browser isn't automatically signing you back in to your account. You can use the same email address for your child's return, but the username must be different and available.
https://turbotax.intuit.com/personal-taxes/online/free-edition.jsp