Note that this procedure can only be done in the parent profile.
Access the My account section by clicking on your avatar in the account header
Locate the field named Password and enter your new password.
Click on Modify my password to save the change.
Remember to save the information once modified.
A friendly reminder: the MY ACCOUNT button also allows you to:
Change your name and email (parent account)
Change your character (all accounts)
Change your language and wallpaper (all accounts)