To reject that there are no innocent children is to reject the doctrine of Original Sin. Do you reject the doctrine of Original Sin?
St. Augustine based his belief in paedo-baptism on the reality of Original Sin being inherited by each newborn from Adam through the parents, thus no children were "innocent".
I do believe that Calvin taught that children who die before an age of accountability (whatever that is assumed to be)are of the elect and will be in heaven. I know that my pastor and I had some disagreements about this. He felt that it was extra-Biblical. And it may be. I am not sure which way I fall on this.
And thus a just God can kill babies according to Calvinism.
This was my point.