I learnt to guard against this particular problem when I found Timon (aged seven) looking pale in his room in the morning having watched his clock for the full night from the 6.30 switch-off. I was quick to compliment him after I let slip the word "obsessive". Remember, one negative must always be followed by three positives. Example: "Timon, your TV-watching has become obsessive." Take a deep breath here, all you mums. "Timon, you have GOOD powers of tenacity, you are VERY GOOD at pursuing a goal." Lastly, with a hug: "Tim, you're a kid who knows what he wants."
For the "smalls" who can't tell the time, I recommend one of those fantastic bunny-face alarm clocks whose ears pop up at allotted times. But do remember to put the clock out of sight every now and then. My daughter Esme, then aged three, spent two hours watching for the ears. When they didn't come up, she became understandably frustrated and presented me with the ears, having smashed the clock to pieces. I remembered, after feeling a bit negative myself (these clocks are not cheap!), to tell her that she was (1) sensitive, (2) resourceful, and (3) physically forceful. As a bonding exercise we went together on a special trip to a watch-mender, and when he laughed in our faces, we went together to buy a new one.
Once you have set the rules, don't be vague about how long they can watch for. Tell them they can watch for an hour and a half and put the clock on the top of the television so you can show them that their time is up. However, it is advisable to put the clock out of reach since the children, as they get older, might be tempted to tamper with it. I was going through a particularly bad patch last year - depression, mild tranquillisers, more depression, suicide attempt - and had to go on strong medication. The children managed to keep the clock at 5pm all night. We'd talked through my illness and discussed how we should help each other and they were obviously being sensitive to my condition. I wagged my finger at them when I found them in the morning, but I was touched. Who says children don't have lovely, lovely natures?
The reason I recommend a full hour and a half is to incorporate the whole of a movie. Children must see those crucially happy endings. The negative role models have to get their just desserts; positive role models have to be seen finally establishing themselves in stable relationships. Beauty and the Beast was a great breakthrough with the bookish Belle turning down macho heart-throb Gaston ("Gastarn") in favour of the ugly Beast. I always give Esme a nudge at that glorious moment when Gaston falls screaming to his death.
I once made the mistake of switching off Pinocchio when it was 10 minutes in. Obviously, I had to lie on the floor to reach the controls, thus putting myself at some disadvantage. Timon (then aged three) jumped on me, grabbed clumps of my hair in both hands and screamed "I hate you Mummy". I managed to say "I love you very much Timon", which is the correct response for mothers in this situation. I eventually persuaded him to help me attach the clumps of hair back to my head as a healing process. We used Sellotape but I don't recommend this since it proved rather painful. However, it was right at the time as it had been Timon's idea. Never, ever, pour cold water on children's ideas or you run the risk of fostering an aggressive streak.
Treated properly your television can be a family friend. My children are never short of material for their fantasy play between sessions. They adore re-enacting the climax of the fight between the "good" lion and the "bad" lion in The Lion King. Esme - the good lion - hangs by her hands from the bars of the top bunk bed; Timon - the bad lion - stabs at Esme's fingers with his claws until she falls down the perilous cliff to her death in the path of stampeding buffalo. Obviously, I've suggested that Esme occasionally plays the more dominant lion but Timon reacted very badly and I haven't liked to mention it again, for violent parent-child interaction is not productive!
Kids can feel flat when you switch the video off. To guard against this, plunge them into other events! As you point to the clock and turn the switch, outline several new plans. How about a cruise around Blockbusters to find a brand new video or, better still, another special trip to the cinema to see 101 Dalmatians?