I’ve never heard of these before. I looked up a sample card. I have to say that the subject of the sample card that I saw is not something I would try to approach with a six year old. The sample presented an objection to objective truth, and then tried to gave a simple answer. But the answer was somewhat circular and IMO not simple or concrete enough for a first step into that argument for a child.
If the sample I saw was like the rest of the cards then I would not feel comfortable using these with my kids. It has somewhat of a Baltimore Catechism feel to it. Those types of questions, or objections, however you want to frame them, are much too complicated for such a framework as question and answer, or flash cards for defending the faith. There’s risk in oversimplifiing answers to theological questions in such a way that presents the answers as definitively simple and straightforward. It’s not a bad starting point, but IMO a child’s mind should not be exercised in this particular way when it comes to this sort of subject matter.