I'm sorry, I don't understand how the fact that possession itself is criminal is relevant. If the government actually knew of the existence and location of illegal files on the case, the foregone conclusion doctrine would allow them to compel decryption (as in U.S. v. Boucher). The same would be true of your hypothetical murder evidence.
If you cannot decrypt the files, can you be said to be in possession of the child pornography? In other words, even if the files are child pornography, if he cannot decrypt them himself, it isn't clear if he is guilty of the crime. OTOH if he can, it is clear he is.