Trying to get my father in law’s phone to appear on my mother in law’s iPad under Find My Device but they have their own individual Apple IDs. Is this possible? Or do both devices have to be on the same ID? Sorry I hardly ever use Find My Device.
You need to set them up as a family. Then it should work if find my "device" is enabled on their devices.
See this article for more details. It also has links to articles for setting up family sharing and stop sharing location with family. Which you may want to look at if nothing works and everything is on.