Three excellent answers that demonstrate the flexibility of linux. And it's always a good idea to be familiar with basic command line operations. Personally I prefer a hybrid approach and I try using mc (midnight commander) when it's available.
sudo apt-get install mc
will get it for you. Surviving tyrannosaurs like me might notice the resemblance with Norton commander from the DOS days. Its operation is quite intuitive. You have two panels. You switch between them with TAB. There are Fn key shortcuts for basic operations, like view, edit, copy, move, rename, delete, etc.
In your case, all you have to do is type mc in your terminal window and navigate to Parent folder in one panel, child folder on the other. Then select parent panel and press + and then enter. This will select all files and folders in the panel. With arrow keys go on top of Child1 folder and press Ins key. This will de-select that folder (Ins key toggles select status of an item.) Now press F5 and all selected content will be copied to the other panel (which is inside Child1 folder.)
I am providing this information only for the sake of completeness and to further demonstrate the variety of solutions available. By all means, go on and learn the basic linux commands and their variations. You can always rely on them being available at your fingertip on any system.