When you log into the game, you should see the Select Character screen. Here, you will see a silhouette with a + icon on it. Select this to create a new avatar for your child.
Please note that family accounts allow up to three avatars, so if you already have three avatars on the account, this option will not appear.
If you would like more than three avatars on the same account, please contact our Customer Care team and we will be able to add up to 2 more avatars (for a total of 5 avatars) to your account.