Those little baby flashcards that go right onto the keys are a neat idea. I've never seen that before.
I use flashcards, but what I find works even better is to print out a line of music and have the student write out the letters underneath. The notes are arranged in steps and skips, so the student can see as they're writing in note names what the patterns are. What I mean is that as they're writing, for example, C D E F G they can visibly see that the notes are going up - line, space, line, space, line - and the letters are going in order. When they write out skips, C E G B D F, they'll see that as they skip spaces and go from line to line the letters follow the "order of skips" (see Bernhard's comments in this thread
https://www.pianostreet.com/smf/index.php?topic=2713.0). With my students, so far, I am drilling them on steps and skips. Eventually, I'll want them to recognize other intervals and chords readily as well, but I suppose that is for later once they get the basics down.
I have them fill out these papers at each lesson (I start with one measure at first and the amount they do gets longer, but the time they spend remains the same, which is minimal) until they can do them quickly and easily. I think it's better than flashcards because they're not only having to figure out each note one by one, they also begin to notice how the letters, lines, and spaces all relate to each other. And having to physically write things out engages them more in the process. It's a good idea to do the reverse, too: give them the letter names and have them draw in the notes and bar lines themselves.