You can do what you're asking but you can't do it from your iPhone or in a Family screen. You'll need physical access to their device.
It sounds like you already have a feel for how to use Screen Time but you can use the videos SravanKrA posted as a reference just in case you need more info.
The short version is you'll need to setup a Screen Time passcode on their device and also setup what limits you want.
By adding the Screen Time passcode, that keeps them out of the settings to change it once you hand the device back to them.
Since they are over 13, they cannot ask for more time from their device so remember they will be locked out of whatever you specify until you can get ahold of the device to change settings. You won't be able to make changes from your Apple ID/iPhone