If you are looking for the assembly that uses a particular child part, open Design Manager on that part and run a "where used" and check the "assembly" box for the process options. Make sure you also give it a folder to start from that is higher than your parts and assemblies so DM can look down through them.
I am not looking for a part that it's used in a concret assembly, I am finding a child assembly that has been created from a family table of a parent assembly, so appearently, once the child assembly has been created, there isn't any link between them
Although, the parent assembly can be inserted in a assembly, and pick the child in question, into the .dft environment, in the boxes appear the code and the description of the parent & child assembly.
To solve that, I can create a child assembly, as a independent assembly. Then I insert the assembly into the .dft, and the data it's shown properly.
Nevertheless, there is no way to know which assembly was created from a parent assembly. That is the problem.