When you first create your account you should be prompted to create your first child. If you somehow missed that step or would like to create more simply click on the MENU tab on the right-hand side of the app. Next, click on the YOUR KIDS button and then click on ADD A CHILD.
You should arrive on a screen with four text boxes. The first is for inputting their name, the second is for their username, and the final two are for their passwords. Once you've filled that screen out click the 'next step' button. The final step has you adding a profile image for the child account, their birthday, and their gender.
Once complete, click the 'all done' button and you'll have created a child account.