I agree with Jashugan
Newgrounds's not a kid site. Pencil kid is probably.
Yeah, a bit disturbing, but it's quite suitable for warned people, and I think as soon as it's written somewhere "Contain mild violence" or something.
For the title retardo, well, people who are mentally retarder could be offended and young kids could obtain a nasty vocabulary there, but as I say, for teenagers and young adult, the term "retard" won't really shock anyone who says "fuck".
Yes, it's a bit gory and vulgar for a kid's game. But it is still well done and not excessive for a more mature/warned public.