This isn't a "legal" answer, just what makes sense to me:
The overall decoration is for an adult, so the entire thing as a whole shouldn't need to be tested under CPSIA.
But any parts of the whole that are then used on children would have to pass.
It seems like the right thing would be to keep the original tags & labels on the items, & save the original packaging. Maybe hide it on the inside of the cake- I don't know how they're put together- so that they're available when it's taken apart and used.
That's if you're buying the things and not making them. If you're making cloth diapers or something for it, that would probably be exempt from testing but would need to be labeled.
...and cute as they may be, I just don't think the words "diaper" and "cake" should be used in the same sentence. bleh. makes me glad I'm on a diet. :-)