Copy files from a directory to a sub-directory (excluding the sub-directory itself)

I have created a folder (directory) named Parent which contains several files (namely file1file2 ... file100) and subdirectories (Folder2, Folder3 etc) and a particular sub-directory named Child1 which itself may contain files and folders. Now, I want to copy all the contents of Parent excluding Child1 into the folder Child1.

The final content of 'Child1' should be something like this:

 Old content of Child1 file1 ... file100 + Folder2 Folder3 + etc.

How can I achieve this?