What are some of the bad habits which affect kids' (3-8 yrs) eyesight? How can parents ensure good eyesight for children.

Most of the people already mentioned about how eyesight can be affected. Although restricting is important, sometimes they might not work or have the desired effects you want. I would rather have the kids pursue different activities like sports, music, arts of their interest. This will positively affect them and develop self confidence that is very crucial at a young age. Basically, they would have less time for TV and other stuff that you are worried about.