Except the situation where the user or password are wrong (in which case you use the "forgot password" button) and the application shows the error "login failed", we will explain below a potential solution to this problem (applicable to Android operating systems).
Step 1: Access the phone settings and then enter the application list ("Settings> Applications") and find the Kinderpedia application.
Step 2: Select the Kinderpedia application and then select "Storage".
Step 3: After selecting "Storage" select "Clear storage" and confirm the deletion.
Attention! Android interface varies from one device to another but the principle is the same, the application data must be deleted.