Pre-download everything now.
For quicker windows updates - at least turn on the infinite cache age mentioned here on your PC.
Ensure under Windows update, advanced: get other MS updates = yes.
Delivery Optimization should be LAN only, usually default.
Then just keep doing your updates and they’ll be kept in cache for when you start on those two.
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Set up your steam library as a network share.
Then start the download on their PCs, pause it and then just copy the files from your PC. Get the downloads at LAN speed instead! Run a verification and carry on.
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If you plan on putting extra drives in those PC’s get an external drive dock and pre-load them. Drive docks are also handy to recover data off a crashed system’s drives via a working machine so a handy tool to have when you are maintaining multiple computers. I have a dual drive one for SATA drives and recently bought a ROG STRIX Arion for NVMe drive work.
Spend more time at Christmas enjoying it!