PS3 question

 Hey, folks. We just got our first-ever PS3 at our house. We've been XBox 360 gamers for a long time, so we're not noobs in general, but just where the PS3 is concerned.

Here's the situation: My kids are loving ModNation Racers. They each created a car and a driver. Now, they want to race their cars and drivers against one another in splitscreen. The game supports splitscreen mode. However, it appears that both kids cannot be signed in at the same time. Thus, only one of them can use his unique car/driver in splitscreen, while the other has to use a generic car/driver combination.

I have my own PSN user account. However, the kids do not. They are simply users on the PS3. 

Can someone explain what I need to do so that they can play splitscreen together and each have access to their own save files/content?

Thanks,

Book