Many parents get very concerned that their youngsters like to sit way up close to the television set, worried that this will damage their sight.
There are several good reasons why kids like to sit to close: the closer you are to something, the bigger it appears. Kids like to deal with BIG images ... easier to see and process.
It also blocks out other distractions in the room when the large screen is filling up their entire field of view.
Unlike you or I, they can actually see very clearly at that distance (this same thing holds true for reading books and writing).
All in all, it is OK for your kids to sit near the TV. If you are concerned that a child may have poor vision causing this behavior, you can do an easy test of this:
Have the child sit on the couch next to you and describe what he or she is seeing on the TV at the moment. Have them tell you about the smallest items visible: the color of someone's shoes or eyes or the number pens on a table, etc. If they can see the same level of detail that you can, you can rest assured that (s)he is sitting so close by choice rather than by necessity.