The naivety isn't him saying it, it's him thinking that such a thing could or would ever come to pass, and what such a thing would even mean if it did.
I'm not begrudging the pope using his pulpit to push for morality (though, he is the head of an international criminal organization), I'm begrudging his frankly idiotic approach. Calling it childlike was perhaps me being kind.
If he was somehow successful, what would it mean? Let's say the whole world got together and signed the "we won't bomb anyone" charter. How long until the next bomb fell? Minutes? Hours?