Also, when sitting at a computer for a long time, a person's blinking frequency decreases, which leads to dry eyes, so I advise you to buy eye drops, like a clean tear and sometimes drip them for prevention
Take breaks, every 45 minutes you need to close your eyes, get distracted from the computer, get up, walk, do light gymnastics, 5-10 minutes is enough. If your eyes are dry, you can moisturize them preventively with eye drops. If everything is okay, just blink more often to keep your eyes naturally hydrated. and, of course, you need to work at a computer in the daytime with good natural light, and it is also desirable to have a protective screen (or glasses).
Just limit the time. Modern operating systems already have built-in parental control, which can easily limit both the total time of work on the computer per day, and the hours allowed for this for child accounts.
You can try to teach them to take breaks from work if there is no time limit.
For less fatigue, you can also reduce the brightness of the screen, increase the font and make a warmer color palette.
Modern screens themselves do not pose any particular danger to vision, contrary to popular belief ... LCD screens have an ideal geometry and image clarity compared to old CRT models and you strain your eyes to make out what is written / shown there is not required.