Why don't we want all children before the age of reason to have Heaven available to them?
1. You're assuming that children who die before the age of reason is a common event. It's not. It's very rare.
2. You're forgetting that nobody dies unless God says so. So if an unbaptized child dies, it's because GOD DIRECTLY PLANNED it to happen, from all eternity.
3. You're forgetting that the Church makes such rules under the inspiration of the Holy Ghost, and thus, such a rule is God's rule.
4. You're trying to understand the mystery of salvation, which only God can understand, who sees all things - past, present and future - at the same time.
5. Nothing in life, no child who is conceived and born, happens unless God wills it to happen.
6. Every soul, even those of pagan children, are dear to God and such are created in His image and likeness. He has a plan for all souls to gain heaven.
7. Salvation is a spiritual mystery which none of us can understand. Some of the Church's rules are also mysterious.
Even an illicit Baptism is valid for the child. Is infant protestant baptism, ( which we mostly accept) that much different than an infant pagan baptism?
Protestantism has only been around for 500 years; in the course of history, that's not long. The Church accepts Protestant baptisms as valid, but that doesn't mean She's happy about children being brought up schismatic and heretical. At least Protestants teach children about God, and some of them are good about following the 10 commandments. A pagan upbringing is MUCH worse than protestantism, because atheism is MUCH worse than protestantism.