Best practices for syncing two Macs using Sierra

I have three children who share two desktop Macs. Each kid has her own account on each computer, but up to now none of them has had an Apple ID (they do not have phones or mobile devices, although they do have old-school digital cameras). Until recently the computers were both running Mountain Lion, and keeping each kid's personal files and photos in sync between the two machines was an ongoing headache involving USB thumb drives and lots of manual copying.

So I've decided to take the plunge and upgrade both machines to Sierra, create Apple IDs for each kid, and use iCloud to sync their Desktops, Documents, and Photos. This seemed like the simplest way to make sure that each kid's account on each computer would automatically stay in sync with the corresponding account on the other computer.

I'm wondering if people with experience with this setup (or a similar one) can share any best practices with me before I go all-in on this. For example:

• Am I correct that this setup will work as intended?
• Will syncing of one user continue to run in the background if a second user logs in to the same machine? (Presumably not if the 1st user logs out, but what if they stay logged in while the second account becomes active?)
• If I link all the accounts on both computers to iCloud, will I end up with duplicate copies of files that currently reside on both machines? Or will the syncing process recognize when two files are copies of one another?
• Would it be better to upgrade one machine, link all of the accounts on that machine to iCloud, then do a clean install on the second machine and create & sync new accounts on that machine to iCloud?
• If I notice that files don't seem to be syncing automatically is there a way to force a sync?

I am going to try answering per question, except the first two. So here goes my 5p:

• Am I correct that this setup will work as intended?
• Will syncing of one user continue to run in the background if a second user logs in to the same machine? (Presumably not if the 1st user logs out, but what if they stay logged in while the second account becomes active?)

Yes, for both.

Also in addition, for best practices and keep each user's file separate, create the Documents, Pictures, etc folders inside and iCloud Drive. Now ask each child to put all their stuff in that folder.

If did not do that, it will still be stored inside the local machines' ~/Users/ folder.

If I link all the accounts on both computers to iCloud, will I end up with duplicate copies of files that currently reside on both machines? Or will the syncing process recognize when two files are copies of one another?

Do you mean using the Auto upload feature?

Would it be better to upgrade one machine, link all of the accounts on that machine to iCloud, then do a clean install on the second machine and create & sync new accounts on that machine to iCloud?

If you are moving all of User1's docs to their iCloud Drive, it does not matter.

If I notice that files don't seem to be syncing automatically is there a way to force a sync?

Not that I know of