I once worked for a company that produced equipment used in hospitals, and I can vouch for the issues installing updates as well. Moreover, hopelessly stupid things were done such as hard-coding the hosts file for remote diagnostics, and logging in and running applications as the Windows Administrator account. Furthermore, the hospital IT staff was equally incompetent, in that even if (by some miracle) we wanted to patch the products we had to jump through hoops to do so, and even simple things like DNS resolution were filtered for our devices.

In order to drive a car, you need a government-mandated license indicating a minimum competency level so you aren't a danger to others.

In order to fly a plane, you need a government-mandated license indicating a minimum competency level so you aren't a danger to others.

The right to practice law, become a doctor, and even have a job (by requiring social security number) is mandated by law.

Why are people allowed to create a public safety problem by opting out of "required" vaccines?

Instead of using any existing language (written or spoken), you use mathematics and pictograms. Essentially the same as how they did the drawing on Voyager.

Just start with a basic number system, individual atoms, then a description of radiation, time periods, etc. None of those should change within a few million years.

If you want to give a start and end date, just use a star chart based on the current location of earth with of a few obvious bodies for reference and project it forward.

I'd highly recommend the series So You Want to Be a Wizard? series by Diane Duane for kids in the 8-15 age range, although they read fine for older ages as well. And before anyone asks, I'd recommend them over the Harry Potter series. For older kids (12-18), the Amber series by Roger Zelazny is great as well. There's also the old Danny Dunn series. Finally, depending on the kids' maturity level, go for the classics (Stranger in a Strange Land, A Wrinkle in Time).