I know that there is a school of thought that watching any TV is damaging but I think if you watch the TV with your children and talk to them about what they are seeing it is a very valuable tool. From about 6 months to 18 months my son and I watched Tellytubbies, we talked about it sang the songs and definitely his first two words 'Star' and 'Butter' for butterfly were as a direct result of identifying what he'd seen on the programme in real life. He started talking at 8 months and at his 1 year check when the midwife said he couldn't possibly be talking in sentences he promptly said 'I want a drink' ! So I don't think it harmed his development in any way. I will confess I occasionally used it to 'babysit' while I cooked dinner or ran the vacuum round but everything in moderation eh!
He's now 10 and hardly ever watches children's TV he watchs Braniacs, Top Gear, Dr Who, Springwatch and science, travel, nature programmes.