I don't quite get you there. How could Mary be sinless? Was she virgin borne? What would that have to do with Christs sinlessness? Are you saying that the children die for the sins of the parents, and therefor Christ death was justified if Mary sinned?
Don't get it, you will have to explain.
It works like this.
Christ is truly God and truly man. This we know from Nicaea. What this means is that he is both - at the same time.
For him to be the Son of Man, and truly man, he must be born - of a woman. What this also means is that he would be stained with original sin.
There’s no way around this, unless you state that he wasn’t born of a Virgin, but rather born ‘in’ a Virgin.
The implications of in vs of are enormous. For one, it denies the incarnation. Christ wasn’t truly man, because he did not inherit a human nature. He was God - putting on a human form. It fabricates the death on the cross.
So, if he’s born of a Virgin, and gets his human nature from Mary, why didn’t Christ have original sin? Why is he both linked to Adam (as in the genealogy in Luke), yet is entirely free from sin?
Well, that means that he must have been born of a Virgin who was, herself, sinless. But we know that no one, but God can be sinless.
It all links together. Mary’s immaculate conception, her sinlessness, her assumption (because those who are sinless cannot die), etc. and it all comes about because Christ is free, not just from his own sin, but also from original sin AND has a fully human nature, just like us. He is more man than we, who have fallen since the fall.