Well, if you adding the child, then it most be something else. Here's some things you could try:
| Make sure the scene you are instancing is not hidden by default.
| Check its position, make sure the camera can see it if it was added to the scene
| Add a print statement to your instanced scene on its _ready function. If it prints to the console, then its added somewhere and we just need to find out where.
Other than that, I'm not sure. Normally instancing and calling add_child works, so the fact that it doesn't seem to be is rather confusing. Hopefully we can find out what's happening!