Hi,
I had a similar problem: Parental Control was always terminated after the mobile swiches to standby (display off). So the protection for the kid was gone... (It's a Huawei Y6 with Android 5.1)
I found the reason in the Settings of Android. It's crucial to define the Parental-Control as a "Protected App" to have it running even in standby. Works well - in my case. Hope it helps.
Perhaps check administrative permissions for the Parental-Control also...
Best regards Thomas