Again my friend, you are singling out one verse to build your doctrine on. It says they died in Adam in Romans 5:12-19, which meant spiritual death. It did not say they are going to die later on my friend, but died.
There have been at least 3 places in Scripture referenced in this thread either affirming or alluding that children are saved.
David's child.
Jesus specifically telling us children will be in heaven (and here comes the allegories!)
Paul telling us he was alive before the commandment of the Law.
This kind of doctrine is exactly why I never refer to myself as a Calvinist, and don't like it when people refer to me as one, even though MUCH of what he believed I also believe. But to be fair, I don't know what John Calvin himself believed about children in hell.
There isn't a single reference that I'm aware of that says children will be cast into hell. You have to stretch Scripture to make that fit.AND ignore Scripture that suggests otherwise.