I have to admit, I'm an old fart who didn't watch Sesame Street when I was a toddler (since I was almost 16 in 1969 when this show first aired). But my son watched it when he was a toddler in the late 70's. It certainly didn't do him any harm (Cookie Monster did not turn my son into a fat cookie addict, who'd a thunk it?). In fact, I would certainly credit SS with having given my son a jump on his letters and numbers before he started school, and doing it in an entertaining way. Granted, we lived in a small town, and he saw farm animals anytime we drove from our town to another one for shopping, etc. But SS also showed him that there were people of colors other than white in the world (the town we lived in was pretty much white, nothing else) and that they were just like him. He didn't need to see Oscar the Grouch to know about grouchy people, he knew about them from real life, but Oscar did teach him how to deal with them.
I will admit that by the time my son was watching SS, he already knew the dangers of being approached by strangers and that going off with them could be dangerous (talk about innocence lost *sigh*). He was a happy, healthy, active, inquisitive child, and watching the old SS certainly didn't do him any harm.
My grandchildren, on the other hand, watched Barney (never could understand why they liked him) and Bananas in Pajamas (now that one was cool, even I could see why they liked it). They also watched Teletubbies, which was sorta kinda maybe kewl, from an adult viewpoint, anyway. The point is, they watched those programs, and while they have out-grown them now (the youngest of my son's kids is 9), they don't seem to have done any harm, despite what the oh-so-politically-correct know-it-alls would have me believe.
Yes, what your children see on tv can affect them, but only to the extent that parents let it affect their children. Trying to make children's television "healthy" by making it about "good" food over "bad" food and exercise over reading, all that is accomplished is creating generations of neurotic people who don't know how to trust their own bodies to tell them what is good for them. This does not let parents take responsibility for teaching their children, it dumps the responsibility for so-called "good" habits on the child. If the child doesn't eat this "healthy" food or do this amount of exercise, s/he is not a good kid. That message does much more harm than good, IMHO.