Yes, you cannot add an existing Child item. The popup dialog only shows the child items that belongs to the parent item.
If you want add an existing one, maybe you can do it like this:
1. create a extra field called "ParentUUID" in your child content type.
2. Edit the existing child item in Child folder and then input the value of the parent's UUID into "ParentUUID".
Regards,
NJ Wu
Kooboo3 Fans
http://www.qiyeqiye.com