Watching TV is not bad, but watching too much of TV is bad for children. It is especially bad for two reasons. The first reason is that it ruins the eyesight of children. The second is that the habit develops and reaches such a situation that the child may not be interested in studies, but in certain channels. In both the cases, watching TV becomes harmful in children.
The best technique is to divert their minds to some other interesting activity which may be helpful in their developmental needs. In their free time, it is good to engage them with some puzzles or some other interesting games. Let them play word building if they have a partner. This will help them build vocabulary as well as keep them firm on the educational area.
Also, let your child’s hobby work out during their holidays. Children mostly have the habit of putting themselves before the TV on free days. Before even they plan to watch TV, ask them to draw or paint using their colours which otherwise they don’t get to do as they had to study. The parent also needs to be a part of their activity so that they have their enthusiasm to paint and show it you.  This would work better if there is more than one child to compete with. Kids normally do wonders when they are in a competition.
Every time your kid shows liking for TV, introduce some new but interesting game. They would be excited to play with clay models or some other creative stuff. There are also some building materials or blocks which could be used to build some objects. Parents may sit together with them and help them make something new. No doubt, kids will be excited to learn the new things.
If at all they want to watch TV, make it a point that they watch only for a particular time limit and that too after finishing off their studies. It is the responsibility of the parents to see if the child is firm on the fixed routine. Make them watch some informative programs instead of the soap serials. Let them see their favourite program on weekends when they do not have any home works left undone.