Flashcards alone won't teach non-verbal children to speak because they lack context. They can build vocabulary in typically developing kids, but language-delayed children need to learn how and when to use words first. Ming from Agents of Speech explains why context matters more than memorization.
The second thing is that there's no context to use the words. Let's say I learn a bunch of words and you've learned a bunch of words in your life for a foreign language. And let me ask you, wow, can you speak that language fluently? For me, I learned French for like three years during middle school. Apparently I was pretty fluent when I was like three. But when I got to middle school, I forgot all of it. And all I remember now is how to say I go fishing and I ride a bicycle and I go to the library. The the reason why is because there's no context to use words. If you're using flashcards when you're teaching, languages must be context based because the problem in children who are delayed is that they don't have a problem in vocabulary. They really have a problem in language use and language acquisition. So if you're teaching them through flashcards, you can do this for non delayed children. It will increase their vocab by a lot. And if you've already taught the language functions that you want the child to know, then of course you can use flashcards, but not at the point where they are nonverbal. Okay.
