Child themes can now be found in themes/ under the parent theme. We are updating our zip formats to make updating themes easier for you all and this is part of that process. When you get the theme just follow this process:
1. Go to themes/ under the parent and copy the child theme into the wp-content/themes/ directory
2. Activate the child just like you always have
3. Remove if you wish the themes/ directory totally under the parent once you have moved out the child
This change is done to help with auto installing and the update notifications so has a purpose in what may seem like quite a change.
Not all themes are updated yet but if you can't find the child follow these steps and it should be there.