Reduce the tools to what is strictly needed to have fun with it, and perhaps add new ones depending on what you feel would suit this attractive mechanic you have there.
I'm not really sure that reducing the palette/number of brushes would do much to make it look more like a toy. Unless you mean something else? I don't exactly have a wide variety of tools.
What additions would you suggest? I can't really think of many things I could add which wouldn't take it more towards the appearance of serious painting software.
You think they're not a lot because you're comparing it to Paint, probably, but compare it, instead, to The Incredible Machine, or something along those lines.
Some ideas to 'toyfy' it:
- Make the canvas constantly rotate (there could be a slider to control its speed).
- Reduce the colors to a palette of four or so, which can be changed, and the drawing reflects the changes.
- Every time a region is circled, flood-fill it with a random color.
- Change the color the brush paints with according to its movement speed.
- Place some 'collectibles' strewn around the canvas that play as a bit of a randomized connect-the-dots drawing.