You can update or reset the password for a Child Profile at any time in the Parents Section.
You can access the Parents Section in the game by selecting the Menu icon in the upper-left corner of the Select Character screen. You will need to enter your account password to access the Parents Section.
Select Child Settings to access the Child Profiles, then select the Child Profile you want to update. Select the Edit link in the Avatar Password bubble to update the password for them. You will be asked to enter and confirm the new password before you Save.