Does anyone have an idea where I can get this actors name?

That’s a toughie. I think I would start by writing to the people behind MagazineArt.org, simply because they seem to have a familiarity with Collier’s issues from that era. This is their covers page, and this is their Collier’s profile. The cover pages give artists names, which is a place to begin – though your exact cover is not on the site. At the very least, they might be able to tell you where to look for the model’s name (or are you certain that it’s an actor?).

Edit: It looks like the artist’s name was Charles A. MacLellan, according to this.

I am quite certain his name is Pennywise.

If he’s an actor and you know anything he’s played on go to http://www.imdb.com/ and type in any show or movie he’s been in and it should show a list of actors and actresses and the names of the characters they have played in the particular feature. Sometimes they even have the pictures of the person.

The magazine is dated March 6, 1920 and the boy looks between 8 and 11 years old, perhaps? So, he would have passed away by now. There doesn’t seem to be anything related to who the child is in the magazine’s contents.