When your child turns 13 (or the relevant age as determined by applicable law in your jurisdiction), they have the option to graduate to a normal Google Account. Before a child turns 13, parents will get an email letting them know their child will be eligible to take charge of their account on their birthday, so you can no longer manage their account. On the day they turn 13, children can choose whether they want to manage their own Google Account or continue to have their parent manage it for them.
