We found two issues with your the parents' Lobby Passwords: 

1. Some children had a future Starting Date and their registration status was 'Future Active'. According to the current code, they could not be checked in. Their Starting Date has been changed to today; now they can be checked in. (I have made a note for engineering about this.)
2. Some passwords had uppercase letters. Since entering uppercase letter on some tablet is a little harder, uppercase letters are not allowed in Lobby Passwords. In your case, these were there because your data must have been migrated from an older version. You should be all set now; the passwords have been reset to lowercases only. Please inform the right passwords to the parents.