I am raising my daughter with English and German. We watch some cartoons in German, but unless I buy DVDs in Germany, it is hard to get such materials here in the United States. I don't think they hurt, on the contrary I find them helpful. As has already been pointed out, films made in the U.S. probably sound better in English (although we have watched Kleinen Bär in Germany quite often, I previewed the original English, Little Bear, and decided not to let my daughter watch it--I liked the German voices better).
I tend to believe, however, that cartoons are not as useful as live actors, because part of learning the accent is seeing the mouth movements. If you can get ahold of old American live action television programs on DVD, they might be more useful to you. I have four old Erich Kästner films that we watch, for example. Of course your daughter may be more interested in current hit films.
Finally, simply watching the DVD is rather passive and most people who study language acquistion believe that a child needs to interact with the language to really learn it. So, when I watch DVDs with my daughter (I rarely let her watch alone), I am constantly commenting in German on the action--getting excited, yelling at the characters, asking her about the action, etc. In short, I do everything possible to make watching the DVD an interactive experience. I do this in German even when we are watching English-language DVDs.
My daughter is just over 3 years old (quite a bit younger than yours) and she watches movies over and over again. In her play, she re-enacts scenes and repeats memorized dialog (in both German and English). Her German accent is better than mine.
In short, I think the DVDs are at least as useful as learning songs, and maybe more useful since the visual clues help illustrate the meanings of the words that are spoken. If you are letting her watch DVDs anyways, you might as well let her watch them in English.