Hi,
Welcome to Microsoft Community.
I appreciate your interest in windows 10. I understand the inconvenience you faced. I will certainly help you with this.
I need more information about this issue so that I can assist you better.
1. Does it happen only with particular child account?
2. Have you tried to set time for another child account?
3. What is the make and model of your computer?
I would suggest you to create new child account then try to set to use the PC during the time range.
To create new child account please follow the below link:
Note: It is also applicable for windows 10 as well.
To Allow User to Only Use the PC During the Time Ranges follow the below steps.
1. Go to Control Panel then Family safety .
2. Under Family safety select User settings then Time Limits then Curfew.
3. Select "When can Standard-example use the Pc under this
select Standard-example can only use the pc during the time ranges.
4. To Block hours click/tap on a white box, or click/tap and hold and drag over a group of white boxes and release, to mark it blue for each half hour of each day that you want to block.
5. To unblock hours click/tap on a blue box, or click/tap and hold and drag over a group of blue boxes and release, to mark it white for each half hour of each day that you want to unblock.
I hope this helps.
Let us know the status of the issue so that we can assist you better.
Thanks.