I'm using Parental Control in F-Secure Internet Security. I set a time limit for my child. After the selected time the screen is locked (and that's ok). Then I pressAlt+Tab shortcut and I see the game in full screen mode and I can play it. I tested it with Fortnite on Windows 10 x64. How can I block it?
Your child may be able to bypass the F-Secure Parental Control if he/she has the administrator priviledge on that user account.
You can view how create a local user or administrator account in Windows 10 by clicking on the above link. Once done, ensure that you have configured the Parental Control for his/her user accounts separately.