Not 100% sure what you are trying to do, but your resident is wrong to begin with.
But I think a resident is not needed at as you seem to be taking unneeded steps.
In your example it is unclear where field D is located. I am going to assume in the "Child" table.
I am also having some doubt about what the key field is between the two tables. I am guessing field C since you are renaming something to C
If so, this is what I think the script should look like:
Load [ColA], [ColB], [ColC]
Load [ChildA] as [ColC], [ChildB], D
The end result will be a table called "Parent" with these fields:
If this does not answer it for you, please be more clear about which table contains which fields and what should be the end result.