I doubt it's as simple as saying that a certain minimal distance to a screen must be kept. I suspect that it is more about which point in space your eyes are trying to focus on. Since those VR goggles come with lenses and an extremely high pixel density as well as maintaining high framerates, I don't think they'll put much strain on your eyes at all. But I wouldn't recommend children to stare into a VR headset for too long anyway, simply because it might stop them from actually going outside.