Will using a computer harm my 3- and 4-year-olds' eyesight?
Not about the eyesight issue, at least as far as we know at this time. Adults who need glasses to work for extended times on a computer have different issues than those of a child. An adult's eyes must work hard to take in the images on the screen, as their eyes don't adjust as readily as do the eyes of younger people. Like the body in general, the eyes become stiff as we age. This results in eye fatigue in the computer-using adult but not in the child. Adults who already have corrective lenses may need a prescription that is just for the distance from the screen to their eyes, to avoid neck strain and allow for correct posture while focusing on the screen.
Kids' eyes, with or without corrective lenses, are more flexible. Some companies are marketing computer glasses for kids, but the glasses seem to offer no benefits except to the stockholders of those companies. So don't buy these; instead, invest in a basketball hoop. Whether on computer monitors or TVs, video images actually put the eye at rest, just as they seem they seem to put the mind and the body at rest.
That's the problem with kidsthey need to be active in mind and body, not zoned out in front of a screen. If your kids seem to be straining to see the screen or anything else, get them evaluated by an ophthalmologist who works well with young children. All kids should have a vision screening in the health care provider's office at the 4-year-old visit and some bright 3-year-olds may be able to have the screen as well. Don't skip any of those checkups and be sure to discuss any visual concerns with the provider.