Dear, your professor�s feeling is correct enough for your dissertation question. You should select a specific topic for your dissertation. While doing your dissertation you should focus on the problem and the solution for that problem, so don�t go with all the negative effects of TV and Film because in this way you would be grumbled in many general symptoms related to the personality and class of each set of children.
E.g. children group of higher, middle or lower income family, the effects and symptoms will totally be different. So select few or one concrete problem and start researching the implications and remedies for those problems. In this way your dissertation would be considerable and reachable for completion. Than you could also formalize that which problem is most intensive and frequent according to the subject.
If you talk about the specific problems than you could broaden yourself by considering internet and other media along with TV and Films. And you can find out which media is largely contributing for these problems.
Be careful on the perfect methodology and selection of
Dissertation Titles and Topics, some of the tips could be find out on following links.
http://dissertation.20six.co.uk/dissertation/a rt/632247
http://home.earthlink.net/~gorski/dissertation /method.html