You have a few options on what you can do to limit your child’s usage of a device.

NextDNS has some interesting features for parental controls, as well as some options to try and prevent bypassing it (i have yet to try it though). On iOS it can be installed as a device profile and you can prevent them from being uninstalled, I’m not sure how to go about this on android but I’m sure there’s a way of limiting the child’s access to changing the settings.

Otherwise the built in parental control features on Android and iOS seem to be very useful as well, since you can limit what your child can install, set screen time limits, etc. I am pretty sure you’d want a more recent version of Android however to get all of the features here which may not be possible depending on the budget.

If you just want the ability to call / text then perhaps a dumb phone is a good first step? But a smart phone may be more useful if they’re older.

Some older consoles have very negligible size libraries in modern day terms, and who knows what sources of ROMs will be taken down by lawsuits in the next few decades. I feel like there is some sense in making a complete archive of a systems game library, but for my personal use I usually just download / dump / rip what I need specifically.

Whatever file format I use them in is also how I back them up, I backup my entire desktop’s and laptop’s data to an external hard drive and an online service provider. I’m sure a compressed format would be more space efficient but that would take much more time given my use case.

In the case of my laptop it runs Linux and the filesystem I use supports “transparent compression” (almost all contents of the drive are compressed with zstd), so I’m guessing any of the ROMs on there will have already been compressed as nuch as they can (but I’m not knowledgeable enough on the file format specs)