So, the best I dare to conclude about the effect of tv on children is: it's ... inconclusive.
Fortunately, some people have time to think about it more seriously. Here is their paper. Below is the abstract:
We use heterogeneity in the timing of television's introduction to different local markets to identify the effect of preschool television exposure on standardized test scores later in life. Our preferred point estimate indicates that an additional year of preschool television exposure raises average test scores by about .02 standard deviations. We are able to reject negative effects larger than about .03 standard deviations per year of television exposure. For reading and general knowledge scores, the positive effects we find are marginally statistically significant, and these effects are largest for children from households where English is not the primary language, for children whose mothers have less than a high school education, and for non-white children. To capture more general effects on human capital, we also study the effect of childhood television exposure on school completion and subsequent labor market earnings, and again find no evidence of a negative effect.
But, speaking of tv, of course we should think about what actually is shown on that thing. Because that might make a big tilt on the conclusion. If your kids grow up watching Discovery Channel rather than MTV, probably they will have better grades (so far as school gives more weight to science than to fashion or music).