People are talking past each other because everyone is thinking of wildly different scenarios.
The specific scenario makes all the difference.
Switching the track of an out of control train to hit another innocent child instead of your own.
Following the orders of a terrorist to assassinate an innocent person so they will release your child.
Murdering an innocent child so that their organs can be transplanted into your own child.
Strangling an elementary school and bottling their dying tears to create an elixir to extend the life of your child.
I suspect most people would switch the track in the first scenario, even if they believed it was the wrong choice.
In the second scenario, many people would abstain, but would still be sympathetic towards those who went through with it. The terrorists take partial responsibility for the murder.
Few would go through with the last scenario, and people would not be sympathetic. There is something seriously wrong with anyone who would go through with this one, but that is covered in the other thread.
The third scenario, in which you murder one innocent child to take their organs for your own child, is the one there would be more of a split on.
The crux of the argument for those who are saying they would do this, is that not doing so would be murdering your own child.
This is the reason your argument falls apart.
We can come up with a word to describe “Refraining from murdering and extracting the organs of an innocent child to give the organs to your dying child”, but murder is not the right word. Not by the standard definition, and not by any sensible extrapolation of the standard definition.