In 2002, the American Academy of Pediatrics prepared a study of how many, and which, children watch how much TV between age 0 and 35 months. They found that
A substantial number of children begin watching television at an earlier age and in greater amounts than the AAP recommends. Furthermore, these early viewing patterns persist into childhood.
A meta-analysis in 2005 reported that:
Although one study finds positive associations of language learning with exposure to some children’s TV programs, other studies find negative associations of viewing with language, cognitive, and attentional development.
...and re-affirmed the recommendation...
...that children younger than 24 months of age not be exposed to television.
In 2005, some researchers studying kids-and-TVs as young as age 2-1/2 confirmed...
...the importance of content and program type when describing media effects. At 30 months of age, watching Dora the Explorer, Blue’s Clues, Arthur, Clifford, or Dragon Tales resulted in greater vocabularies and higher expressive language scores; watching Teletubbies was related to fewer vocabulary words and smaller expressive language scores; watching Sesame Street was related only to smaller expressive language scores; and viewing Barney & Friends was related to fewer vocabulary words and more expressive language.
In 2004, some researchers reported:
...hours of television viewed per day at both ages 1 and 3 was associated with attentional problems at age 7...
Early television exposure is associated with attentional problems at age 7. Efforts to limit television viewing in early childhood may be warranted, and additional research is needed.
Also, in 2005 a large study found that
There are modest adverse effects of television viewing before age 3 years on the subsequent cognitive development of children. These results suggest that greater adherence to the American Academy of Pediatrics guidelines that children younger than 2 years not watch television is warranted.
So without knowing the exact way that TV affects a child's mind, it is clear that TV does have an effect.
Watching educational programming improves scores on specific metrics when compared with cartoons, but it has been demonstrated again and again that tiny kids who watch too much TV become bigger kids who watch too much TV, and those kids fall behind.
The exact mechanism by which programming may affect a child's mind has been explored somewhat in controlled studies. Three such studies were summarized in 2015:
Three studies examined the short-term impact of television (TV) on children’s executive function (EF). Study 1 (N = 160) showed that 4- and 6-year-olds’ EF is impaired after watching 2 different fast and fantastical shows, relative to that of children who watched a slow, realistic show or played. In Study 2 (N = 60), 4-year-olds’ EF was as depleted after watching a fast and fantastical educational show as it was after a fast and fantastical entertainment 1, relative to that of children who read a book based on the educational show. Study 3 (N = 80) examined whether show pacing or fantasy was more influential, and found that only fantastical shows, regardless of their pacing, disrupted 4-year-olds’ EF. Taken together, these studies show that 10–20 min watching televised fantastical events, relative to other experiences, results in lower EF in young children.