So you're saying there is no wan to measure it with the router... sad.
This just is not a feature of network router. Is it suppose to identify Mario Kart or Doom too?
You can restrict a device (say Xbox, PS3, etc) by time-of-day. It's that you want to restrict by application that makes this problematic. But, RouterOS is very flexible, so if you want to research what network IP/ports it uses, you can do something. But the issue is modern games use a variety of NAT traversal methods, so it just not easy to find specific application. This isn't "sad" — it just how networks and routers work.
Assuming you know the IP/ports/etc that make up "Fortnite traffic" (and that's a big IF since other games may use same ports and/or Fortnite uses many domains/IPs...)... the "in time" part of the request here can be solved with /ip/kid-control or some network monitoring tool (like Dude or third-party).