The best thing is to have the contrast/brightness set right to the rooms lighting conditions. In a dark room a tv does not need to be very bright but in a sunny room, like with a large south facing window, it will need more contrast/brightness.
Plasma has a more "real life" quality of picture, LCD is more cartoonish (vivid) on the colors. If this is just for the kid, you should take into account the kids age, are they going space on something put it on pause and forget it for hours, day after day? Yes plasmas have a built-in screen saver but it is limited to what is can do.
as subject i have a question.
which tv is better for eyes (especially for child)? plasma or led . and why ?