Just a regular Joe.
What is the typing experience that you want, and for which language(s). It’s not clear to me, sorry.
It is possible to map keyboard input in various ways. For more complex use-cases, many programs support character substitution as you type (eg. gx could become ĝ automatically).
Additional SPoFs: Your upstream internet connection, your modem/router, electricity supply, your home (not burning, flooded, collapsed, etc.). And you.
You have an opportunity. Give him a pre-installed Linux and a terminal, along with a page of commands that he can run to do neat things… including starting the GUI to watch his favourite (ideally pre-downloaded) videos, running some demos, etc.
Don’t make it too easy, but not too hard (2 you said? Can type a few characters though…)… Add to it over the years, unlocking the power, and guiding him to discover more by himself.
Kids won’t become tech savvy if we hand everything to them on a silver platter, with touch screens, controllers, and flashy games. It can be bland and boring, until they do something.
It might just be the most life changing gift they ever receive.
Welcome to the world of Carrier Grade NAT. 100.64.0.0/10 is reserved for this.
If you are lucky, you also have an IPv6 address. The catch is you need IPv6 on the client-side too.
A VPS or similar running wireguard and a proxy might bridge the gap.
It might also be possible to ask your provider for some port forwarding. Probably not, but check anyway.
Good luck!
My pranks were less destructive …
/ctcp nick +++ath0+++… it was amazing how often that worked. 🤣