I'll try to answer your first part, or at least try to offer some help about where to go for more info. But first I have to give a spiel on why these studies are problematic. The studies I can think of off the top of my head are mostly about slightly younger kids. You could look at the authors, what depts they are in, what journals are publishing this material, and the keywords for their articles, and probably a few google scholar searches would turn up more scholarly articles on the topic.
Are you affiliated with an academic institution? If not, it may be hard to get free access to online scholarly journals in which academic studies (esp recent ones!) are published. Though you might be in luck b/c this is a topic with wider appeal, so some studies may be published and presented for the public as well. I'm a PhD student in a communication department and as such have come across some articles about children and screen media- but its NOT at all my area of study (though I'm also a mom of a tv- and dvd-free 4 year old, but it was a decision we made as a family, not one I made as an academic, ykwim?).
One problem with quantitative studies of "media effects" is that its *very* hard to disentangle all the factors and measure the effects on any particular child. Many factors you've mentioned complicate matters- like, are the parents watching with the kids, what else is going on in family life, basically the bigger picture of the child as a whole person. The "testing" is often done in a laboratory setting, so, obviously not very realistic to normal conditions, or else it is based on parents reporting of children's viewing habits and behavior, which would present a different set of problems with measurement and aggregating data.
Its hard to know the causal relationships here- for example: say a study finds an association between television viewing and poor attention skills in children. BUT, 1) does tv cause poor attention skills, or 2) perhaps parents of kids with poor attention skills resort to more tv to manage their kids because the kids can't sustain an activity for an age-appropriate length of time, or, 3) maybe parents with poor attention skills also have kids with poor attention skills, and those families also have tv going more often to provide external stimulation, but tv-viewing wasn't strictly a cause or effect, just an associated behavior, or 4) the study size was too small, or the researchers were already biased about what they would find, or there are factors they weren't considering that were stronger predictors of attention, etc. (just to be clear, I'm not suggesting any of these things are correct, just rattling off the top of my head some ways you could interpret a correlation between quantitatively documented poor attention skills and tv-viewing).
Also, this kind of research often doesn't account for the fact that screen media is not a strictly "one way" phenomenon- the viewers have to decode or interpret the meaning, and thus its not necessarily uniform in how it is "read" by the viewer.
Here is a link to an article in Time magazine that reviews an academic study that made a bit of a splash, looked quantitatively at Baby Einstein and found it actually delayed language development:
http://www.time.com/time/health/arti...650352,00.html
If you do a google scholar search on the lead authors in the study referenced in that TIME article, you will come up with a lot. You can even look at who cites their articles if you want to find things that may refute or challenge them.
Frederick Zimmerman, Dimitri Christakis
Here is a link to an article with a similar point:
http://abs.sagepub.com/cgi/content/abstract/48/5/505
An article I found personally kind of interesting which just got forwarded to me as part of some seminar I wasn't even in: "Young Children's Use of Video as a Source of Socially Relevant Information" published in Child Development, Vol 77 No 3 (2006). Here is the text of the abstract, which might be enough for your purposes: "Although prior research clearly shows that toddlers have difficulty learning from video, the basis for their difficulty is unknown. In the 2 current experiments, the effect of social feedback on 2-year-olds' use of information from video was assessed. Children who were told ''face to face'' where to find a hidden toy typically found it, but children who were given the same information by a person on video did not. Children who engaged in a 5-min contingent interaction with a person (including social cues and personal references) through closed-circuit video before the hiding task used information provided to find the toy. These findings have important implications for educational television and use of video stimuli in laboratory-based research with young children."
Hope this helps. There is a lot out there, I'm guessing your challenge will be sifting through it, not finding it, once you are looking in the right places.
-Emma
