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What we can say is ALL are by nature "children of wrath". We can also say that a "remnant" of mankind is saved by grace. And we can say that the elect are those who believe upon Christ. Anything more is basicly wishful thinking; a hope that infants are included. And even this hope must be undeniably restricted to the infants of believers. Should God save any others, it is by grace. But whether He does has not been revealed in Scripture.
I agree that we cannot know for sure what happens to babies when they die, but I would take the hopeful view that David's baby went to heaven and that this was the comfort David was given in his sorrow. That doesn't necessarily mean that every baby of David's would necessarily go to heaven because they were his children. There is strong evidence that neither Absolom nor Amnon were elect.
Why should this hope be limited to the children of believers?
God saves His elect from non-Christian families every day, doesn't He? And wouldn't it be all of grace no matter who it is that He elects?