Go to "Schedule a call", and tap the button below "FOR CHILD:".
If you have previously setup one or more children, you're taken to the children list. Press the green round button in the bottom right (with the "+" on it) to add another child to the list.
After filling in all details of a child, make sure you press the green round button with the "+" on it in the bottom right, in order to save the new child's details.
If, while adding a new child, you instead press the back button in the top left, the child is not added and the entered data is lost.