Well if the kids are keeping up with the current cartoon, yes they know the Shredder's gone. They also know April does some martial arts.
Counting the first movie and both series, that's already three times their origin's been told. An ultra violent origin story would be cool, but as long as it's still a viable franchise for kids as well as a currently running TV show, I wouldn't count on it. I haven't seen this new movie yet and the only thing that gets me is that the plot of the movie sounds kinda lame. Anytime I hear the words "ancient warriors" together I have to heave a giant sigh.