Concept artwork for a 2d iPhone/iPad game

Greetings artists!

I am prototyping some computer game ideas and I need a good illustrator/concept artist to inspire the development team visually. I am looking to get concept artwork done that is for a game designed primarily for young children.

Here is what I need:
I need concepts for four types of characters. Bubbles. Think of a Bubble being similar to a Pac Man style character that moves across the screen. Each Bubble has a specific persona: HERO | POWER UP | VILLAIN | BOSS. The POWER UP Bubble will have 4 types.

So you will need to do concepts for 6 types of Bubble characters - each with its own distinctive style and persona.

You will need to have the following to successfully bid for this:

a) Previous concept artwork in your portfolio that shows innovative character designs

b) Previous work doing concept work for a game or iphone app highly favored