Note that craft knifes of this size are generally reckoned to be disposable, and new are really sharp, but don't hold their edge well. I prefer a proper scalpel (or second best an X-Acto type) where only the blade is replaced but I've had a few where a scalpel blade was permanently moulded into a plastic handle, and it does seem rather wasteful to go chucking them out.
This would be a good opportunity to practice with a stone, which is quite easy on a straight blade. There are a few options, but I'd choose a double-sided (coarse/fine) oilstone, just a cheap one to start with.
The hardest part is getting the tip really really sharp, which is an issue if you're using the tip to cut out stencils (just as an example) but completely unimportant for sharpening pencils (extreme example in the other direction).