We just can't underestimate the influence that friends at schools have on kids once they attend school.
Though my boys don't watch Ben 10 and Transformers or any of the other cartoons/programmes with fighting themes, they will still meet kids in school who do.
A few months ago, my 5 y.o. started getting fascinated with the idea of pretend play with guns and shooting. Since there is no toy gun at home, he makes his own with tiny Lego pieces.
So now, even little Chipsy will create his own Lego 'rifle' and run around the house playing fighting games with his bro'. They pretend to 'shoot' every moving thing and each other with a loud 'geng-geng-geng', which is frankly very noisy, but yet funny to watch.
As much as it annoys me, I *try* not to interfere, though I made it clear that mummy doesn't like it when they play like this. 'But we are only pretending, no one gets hurt here!' is the reply I always get.
Fingers crossed that it is a phase and they will soon grow out of it.