if all humans have sinned then how can you call babies innocent?

The person has to know that they are doing something wrong for it to be a sin or else the person cannot be accountable for it. You'll see this in the book of Jonah. The people didn't know their left hand from their right. They didn't know they were living in sin according to the Biblical record of Jonah, and this is why God spared those people.

The proper understanding of that Scripture is to read it in context. Paul was writing to the judaizers who thought the gentiles needed to be circumcised in order to be saved. Paul writes to them saying no because both Jews and gentiles alike are sinners and both are saved through faith and not works of the law. So circumcision, a work of law, is not required. But faith in Christ is.

Paul writes that all have sinned without distinction between Jew and gentile. He doesn't say all have sinned without exception. He couldn't say that. Jesus is an exception. Infants are exceptions. And Catholics believe the Virgin Mary by a special act of God's grace is an exception.

"For there is no distinction; ¶ since all have sinned and fall short of the glory of God, ¶ they are justified by his grace as a gift, through the redemption which is in Christ Jesus, whom God put forward as an expiation by his blood, to be received by faith."

- Romans 3:22-25

Babies may be innocent now but will grow up to be hellish teenagers.

the screaming babies at walmart, everyone in heaven will be childlike

You cannot according to the bible