When you register a copyright with the US Copyright office, there is space on the form (paper or on-line) for multiple authors. For example, here's the paper form: http://copyright.gov/forms/formtx.pdf There is a place to fill in just what each author's contribution is. Like you could say, "Alice Smith", "text" and "Bob Jones", "illustrations".
You can find plenty of books that say, "Copyright 2010 by Alice Smith and Bob Jones" and the like. Lots of books have multiple authors.
Does CreateSpace have an option for Amazon to register the copyright for you? I've published two books with Amazon and in both cases I registered the copyright myself. If there's an option for that and you gave only your own name and not your co-authors, that's a mistake. Note that getting an ISBN is not the same as registering a copyright.
In any case, nothing on a copyright registration says how royalties are to be divided. As TrskalJM says, you should discuss the division, write up a contract, and all sign it. You say you have a written agreement about the "narrator"s portion, anyway. Does that also specify the division between you and the illustrator? If not, you should write that up also. Putting things on paper can prevent confusion and arguments down the road. "I thought I was supposed to get 35%." "No, we agreed you'd get 25%." "That's not what we said last year." Etc. People forget exactly what was said, interpret words differently, make assumptions, change their minds about what's fair, etc.
If the "narrator" hasn't actually written any text, just told her story and you wrote it down, she wouldn't have any claim to copyright. The actual author and illustrator certainly would. But as I said, copyright registration doesn't specify division of royalties. There are all sorts of reasons why someone not named on the copyright registration might get a share of the royalties. That's why you need a written agreement.