In the Sdui app, click on "Account & Settings" in the drop-down menu at the top right and then on "Linked Children". On the next page, select the child whose password you want to reset.
Scroll down until you find the line "Password" and click on "Change" to the right of it. To be on the safe side, you will now be asked to enter your own password and then create the new password for your child.
If you click on "Save", your child can log in with the new password. As soon as your child has logged in, the password can be changed again in the same way.