You are seeing the lock as there have been assignments set to "Mandatory", which prevent children playing other games until the assignment has been completed.
If you would like to turn the mandatory requirement off, click on your Premium user ID and select Assignments from the dropdown. You will then see a list of "In Progress" assignments, and if they have been set as mandatory you will see a lock and "Yes". Click on "Yes" or "No" to switch the setting.
