My use case is I want my child to grow their native vocabulary, eg. when they read books with new words that they don't know. I couldn't seem to be able to pair the languages that way. Is this supported?
I strongly suggest using Anki. It’s free unless you want to use the iPhone app and cloze cards are exactly what you want if the goal is to be able to use new vocabulary correctly. You can even add dictionary definitions in the Extra field.
All that said the best way to increase native vocabulary is to read more.
Absolutely! This is actually one of the key differences from other SRS software. Ie. you can search the central DB for flash cards and just "add them to the learn queue" or "Claim known". Check out the video on my landing page at 4m:40s for how to do it.
All that said the best way to increase native vocabulary is to read more.