Pulled from /. post from "by williamyf"

"put Win10 or linux on the machine, virtualize Win7 (this will help you harden it), install all the patches (including the paid ones from Jan 2020 until Jan 2023, it can be done), then install 0patch ( https://www.0patch.com/ [0patch.com] ), this will buy you some time."

The other machines, you can bring to Win10 as is (do not forget to debloat/defang/decrapify). But not to any version of Win10, instead, go to an LTSC, either LTSC2019 (supported until 2029), LTSC2021 (supported until 2026, yes shorter) or IoT2021 (supported until 2031). As a matter of fact if you have 3 machines, you can try each one and see which one interacts best with your particular SW."

"Meanwhile, in one of those machines, you can try different ClassicShell like programs to try to make the machine behave as close to possible as the way your ASD child likes it. Once you nail it, you can migrate his machines from Win7 to Win10LTSCxxyy
Best of luck."

If you are running Win7, chances are high you can run Win10 as well. If you are running Win 8.1, is certain that you can run Win10. So, your first line of action is to go to Win10, but not any Win10, instead, go for LTSC 2019 (supported until 2029), LTSC 2021 (supported until 2026, yes, shorter than 2019) or IoT 2021 (supported until 2031). Decide which one based on compatibility with your software, with lenght of suppot being the tie breaker, not the other way around.
If you CAN NOT go to Win10, say, because some driver/SW can not be coaxed to run on Win10, or because Win10 lacks a feature like the HTPC bits present in Win7/8.1, one bet is to stay on Win7 (or downgrade from Win8.1 to Win7), virtualize the environment on top of a fully supported OS (this will help with hardening), use PIC/USB passthrough for any recalcitrant hardware, apply all the Win7 patches, including those released during the 2020-2023 paid support period (there are ways to do it), and then install 0patch ( https://www.0patch.com/ [0patch.com] ) While this is by no means a perfect solution, is the best that can be done under the circumstances. Please, do not use that VM for general purpose computing, only for the task(s) that can not be moved to Win10 and/or Linux HOST OS.
If neither of these solutions is good enough for you, then explore the wonderfull world of linux or BSDs.
Myself, for my older machines (where older machines means 2015 and 2018 Macs that can not run win11), I am going for one of the LTSCs, with Linux as a fallback. As for other older machines that belong to Friends and Family, agaiun LTSC with Linux as fallback.