How does anyone know that Infants are among the elect?
I don't claim to "know".
What I said was, there is Scriptural support for John Calvin's belief that God chooses to monergistically Regenerate and apply the merits of Christ's Atonement to those whom He permits to die in infancy.
I didn't say I "knew" it, I said that I thought that there's some Scriptural support for Calvin's view.
But there is an alternate view, of course. The greatest Theologian of the Free-Will Protestants taught that all infants who die unbaptized are Universally Damned To Hell. So, you can believe that, if you prefer.