While being logged in with my Microsoft Account I did not manage to create local accounts for the kids. There is this nice and fluffy “Create accounts for your family” system, but that implies that you create Microsoft Accounts for everybody.
I refuse to do that for kids. Sorry Microsoft, they are kids, they should not need cloud-based accounts yet.
Apparently you can create additional local account if you’re logged in on a local account. Great.
But there is no GUI to set the time limits for the kids if they are using a local account. You have to do this one the command line. e.g. with
net user kid1 /times:Sa,08:00-20:00
See this article.