Seeing this is pro bono I will offer some ideas as I enjoy conceptualizing and designing flyers.
I'm thinking all the comic book heroes; Superman, Incredible Hulk, Green Lantern, Wolverine, throw in Wonderwoman, in the background with Spiderman and Ironman prominent in the front saying happy b-day (kid's name)!
Fantastic Four (with the #4 being prominent) since he's turning four. Dr. Richards stretching out his arms with a big cake being centralized in the invitation.
Within the cake is Spiderman vs Ironman fighting.
Place the kids image over Ironman and depict it as a movie poster.
IRONMAN or SPIDERMAN 4 coming to a theater near you. (destination and address of the party.)
Released in all theaters on (date of party.)
Starring (kid's name) as Ironman or Spiderman.
Produced by NM, graphic artist, + M.C. Directed by your supervisor's name.
Rated G. For kids only. Adults not allowed.
Actually yah making it into a stylized movie poster would do it. That idea is a winner. You're boss will like it for adding him on it.
The movie posters itself are drawn out professionally so it'll be less time consuming for the graphic artist. All they have to do is transpose a few images and add in some text.
Marcus Aurelius: Tell me again, Maximus, why are we here?
Maximus: For the glory of the Empire, sire.
Baked, not fried... the healthy choice.