Good question. The issue revolves around the guilt of sin. We need a Savior to absolve us of our sin guilt before God. Unborn children have no sin. Infants have no sin. Now when do children reach the age where their awareness of sin would make them accountable before God is a good question. So far as I know, the Bible never directly addresses it, but there are several scriptures that, at least indirectly, refer to it. Dt. 1:39 says, "And as for your little ones, who you said would become a prey, and your children, who today have no knowledge of good or evil, they shall go in there. And to them I will give it, and they shall possess it." Here are many other texts should you care to look at them. But one way or the other, we are not changing the subject here. The question, for your benefit, is this. What is the moral difference between killing an unborn child and killing a child that has been born?
https://www.openbible.info/topics/age_of_accountability