It says that my child used the device, when it was blocked (Weekly Reports)

This article explains why Weekly Reports may show device usage even when the device was blocked. It clarifies that the report records the first and last time the device was used in any mode, including Lock mode.

 

 

Please note that the first and the last used time shows the time used in any mode, not just Child (even if your child used the device when it was in Lock mode).