So I used the custom instruments hack and I want to make the Maxis Guitar child sized so a child can use it. How can I reassign the usage of the guitar so only children can use it and still gain creativity?

I'm afraid it's not really very simple. Most objects which can only be used by adults do not offer any animations that are the right size or shape for children. In order to make it usable by adults, you have to hack the code to make it work properly, but you also have to build a set of animations to allow a child to look like he or she is playing the guitar.

While it's not impossible, it's pretty high reaching for a first project, and not something I'd really recommend without a good deal more experience.