Without a SIM card FaceTime will only work when the iPad is connected to a wi-fi network.
If you want the iPad to work on your cellular plan, you'll need to leave the SIM card in, and the account active with your cellular provider. Your Apple ID is not necessary. The iPad and FaceTime will work with any Apple ID. The cellular network does not use it to provide service. So, you can have the grandkids (or their parents) set up an Apple ID for the iPad, and assign it any email address they like. It would probably be easiest to use the email associated with the new Apple ID.
This would also let them use and control the iPad for other things. If it's new enough to run the current version of iPadOS (or fairly recent) they can so use the parental controls, if needed.
You would still need to pay for and monitor the cellular plan and its data usage.