Hi Sarah,
Family Safety still works on earlier versions of Windows. You have to manage the devices individually, however.
Two things might help from earlier in this thread -
First, is the PC set up with your son's account set correctly as a child?
Secondly, are you making the changes to the right profile on account.microsoft.com/family?
If you're not using any devices using Windows 10 or a Windows Phone, you need to make the settings adjustments on the *local account* entries only on account.microsoft.com/family. If you're making settings adjustments to the profile that shows her MSA, those changes will not be appliedto your Windows 8.1 devices (even though she may log in using the MSA). Look for the line item with her user name and the PC name.
If there's a particular device that's problematic, also make sure that *on that device* your daughter's user account is set as child and make sure to click on the "manage your family safety settings online" button in that UI on that device. The synching relationship might need to be reestablished. If she's not ever getting the message that the account is being monitored when she uses that device, and she seems to have access to more than she should, this should help.
If this doesn't resolve it, let me know.