This is a darn good question, and I've always wondered why one of the neighbors didn't pick up on this. First of all, playing with cards was not considered a healthy pasttime for children during the 1930s, and Jem must have known that they would have gotten in big trouble had they been playing any kind of card game. Dill's excuse for Jem losing his pants was a quick response to a question that would have gotten the kids into even more trouble had Atticus known the truth--that Jem lost them on the Radley fence while running from shotgun fire. Jem's response to Atticus's question--"Were you all playing cards?"--was also a lie, but one that would not get them in as much trouble as playing cards. When he said, "No sir, just with matches," Jem literally meant that they were playing with matches--lighting matches. Atticus may well have realized that there was more to the children's stories than what they had revealed, since later in the novel Scout discovers that he knew all along that the children were the ones that Nathan Radley had chased from his collard patch. Atticus may have decided to spare Dill any further punishment from his Aunt Rachel by accepting Jem's story about playing with matches. This, however, doesn't explain why none of the other adults questioned the children's ability to play strip poker without playing cards. Perhaps the nature of their game flabbergasted the others so much that they chose not to pursue the subject further. After all, strip poker was not a subject ladies such as Miss Stephanie, Aunt Rachel and Miss Maudie were likely to discuss in the presence of men.
I have seen matches that have playing card hands printed on them. They come in a book, and the card designations are printed on them, e.g. K heart, 5 diamond, 3 spade, J clubs. (They use pictures for the suits). I think you could play some form of poker with these matches. I found the matchbooks I'm describing in my mother-in-law's apartment when we were moving her to a nursing home. She saved things, so I believe they were pretty old, but I don't know how old.