When you first turn it on, it asks if you want it to try to auto fill the kids section. If you say yea, it does it’s best by using genres like children and animation. But it’s worth doing a pass yourself to check what it did.
After that you use the visibility feature to choose if your content should show in the kids section, the main library, or both.
There is no bulk editing yet. But using the web admin would work best. It has a way to filter by visibility.
A good strategy is to filter by Kids and open anything that doesn’t belong into a new tab. Then go through each tab to change its setting.
You can do the same while filtered to Library.