You have a scripture on that do you not?
No one is in heaven because they deserve to be there..that is the fallacy of your position.
If infants are saved it is because God has mercy on them , not because they are "not accountable" . The bible is full of the judgment of God on the sin of infants .
Happy Easter
A) What sins do infants commit? A couple of examples would be fine. B) If God can have mercy on those who have not received the gift of grace, is He acting "outside of His nature" (as you Calvinists like to term it)? Or is "mercy" part of his nature? And if "mercy" is part of his nature, what is to prevent this mercy from acting over and above his righteousness in other instances?
It seems you have just created a way to deal with the problem of infant salvation, etc., with a workaround rather than cogently dealing with the issue. Just my opinion.