You need to pay attention to clairewait's suggestion above - excessive TV watching gives rise to a lot of the evils above, but we do need to remember that TV by itself isn't necessarily "bad" or "good" - it is how it is used that makes it a benefit or a hazard. I do think TV in moderation with carefully vetted programmes can be of great educational benefit to kids. Just think of nature programmes for example, which my kids love. I guess one danger for parents is when we use TV as a kind of "nanny" because we are too busy or tired to do anything with them ourselves.
The negative effects of television are great. I think the biggest negative effect is the impact it has on social development. Many children sit around watching television and rarely play with other kids. This leads to stunted social development. Kids who spend little time interacting with others have less exposure to positive and realistic conflict management and impulse control.
I think you should probably assert at the beginning of this paper that many of these negative effects are a result of excessive TV. I mean, you can pretty much argue that too much of anything is eventually detrimental to health.
Anyway, too much TV leads to a sedentary, inactive lifestyle. It inhibits children's imaginations. It keeps people inside as opposed to outdoors, which is generally considered healthier.
It's very easy to overuse television as a babysitter, and then it becomes part of a child's socialization. Even with older kids or adults, it is a great "de-motivator", leading to long days sitting on a couch, often eating, while doing nothing to move around. Hmm, kind of like sitting in front of a computer typing on message boards, but that's beside the point. TV often makes light of violence and sexual themes too.
It can also give the impression that one is knowledgeable about various topics, when the reality is that what is presented is limited and slanted.
Mostly, though, it is a big time waster. There are so many things to do. I am always hearing people complain that they don't have enough time to do...something (usually exercise), and it turns out they are watching TV all evening.
In addition to what the first answer said, I would say that too much TV can lead to kids having three other problems.
First, people argue that TV causes kids to become too used to violence. The kids see so many violent acts on TV that they think that violence is normal and acceptable.
Second, you can argue that the same goes for sex. Sex is everywhere on TV -- maybe not actual sex, but people acting and dressing in very sexualized ways. This has led to kids wanting to be like what they have seen on TV.
Finally, it makes kids more materialistic. It makes them want everything that they have seen in commercials.
There are many negative effects of television, especially in regards to children. First of all, children who spend a great deal of time watching television get less exercise. Obesity is a problem in the United States as it is reaching an all time high. In addition, these children receive less socialization with other children. This can be very devastating to a child because good socialization skills are crucial in the development of becoming a productive adult. Television also takes away from family time. This is also true for families who eat in front of the television. Instead of sitting at the table engaged in conversation, they are focused on what is on television instead.
Aside from all the effects mentioned above i believe another negative effect of television is on the education and general knowledge of the child. Those children who have been bought up without tvs tend to be better readers, writers and spellers because they resort to reading for entertainment. This also improves their general knowledge because books tend to be more factual than the television programmes watched by children. Children who read a lot provide themselves a good base for future life and their high school and college education where reading is far more productive than being distracted by what is on the television.