Yes, it is a bad idea if it's excessive and unsupervised. Television is one of the most influential forms of media, particularly for children. The impact of television on children's lives depend on several factors including the number of hours of exposure, types of television shows they watch, presence of adults during watching, and the children's age and personality.
First 2 Years
According to the American Academy of Pediatrics (AAP), children under 2 years old should not be allowed to watch television at all. Even shows that are said to be educational should not be shown to them, as these inhibit their ability to learn through interactive ways of exploring the environment, communicating with parents and playing with other kids. Television watching for babies and toddlers can turn them into passive observers, which has negative effects on their physical and social development. It is important to remember that the child's first two years are most critical for brain development.
After 2 Years
As for kids older than 2 years, the AAP recommends no more than 1 to 2 hours of quality programming a day. When you say quality programming, this would include only highly educational television programs that teach kids good values and cognitive skills.
Television watching for children is not entirely a bad idea if these guidelines are followed. However, if a child is exposed to too much television and bad choices on programs without guidance from the parent or an adult during viewing, the following negative effects can take place.
Rampant violence in television has a negative effect on children. For one, it inflicts the "mean and scary world" syndrome that shoots up their fear about the outside world. It can also cause desensitization to real life violence. When children watch violent cartoons portrayed as funny, this instills the belief in them that violence is something you can laugh about. As a result, children may become more aggressive in behavior, mimicking what they see on the TV.
Another area that parents should be concerned with is the child's sedentary lifestyle. Time spent in front of the television should be spent in more fruitful activities such as reading, playing, participating in sports, learning music and exploring nature. These activities are crucial to the child's early formative years. Television steals the time that should be spent on doing those. Not only does this affect the mental development of the child, it also has negative effects on the child's physical health. Lack of physical activity is a precursor to childhood obesity and other health problems.
Apart from the lack of physical activity, kids are also exposed to bad marketing messages that promote junk food and other unhealthy dietary choices. Most television ads geared towards children are for fast foods, sweetened items, and candy. Only a measly 4 percent of commercials are for healthy items.
Sexual content is also abundant on television. Children may not be getting the right idea about proper sexual behavior as well as responsibilities and risks by watching television shows that have implicit sexual content.