How can I guide users through my navigation metaphor (or replace it with a better one)?

My application is a large suite of educational games for children ages 6-13. There are about 100 games, organized into a kind of map showing top level categories as balls around a "hub":

(I'm obfuscating some of these because of trademarked images; obviously in the app it's not like that.)

You click on a category to enter it. Each category can hold sub-categories, and eventually you get to a bunch of balls that represent the games. If you click on a game, you leave this hub view to play that game.

Our challenge is this: how do we guide kids to certain games and away from others?

For instance, if a kid is weak in fractions, we want to steer him to the fractions games. But those games are scattered in various categories beneath this hub view. Or we want to discourage playing a certain game, because we think it's below grade level, but we don't want to prohibit/lock it, because maybe the kid needs to review that material.

My best idea so far is to reduce the opacity or color saturation or size of the balls that we don't want the player to pursue as much, but I'm not sure how much I like this.

Is there a better way of guiding the player along certain paths? Or, is there an entirely different way to represent this information and help users navigate through it?