I can't help but wonder if both sides are dealing with speculation.
What we know is only elect (infants and older) go to heaven when they die.
We also know that all come to salvation the same way (by grace through faith) and that includes babies.

Saying that all babies who die in infancy were/are elect is speculation and not based on Scripture.
However, as far as I can tell, saying that there are some babies who die in infancy who were/are not elect is also speculation and not based on Scripture.

Why can't we just leave this matter to God and stick with what God has revealed from His Word?

Tom