I think that the "period piece" might be a reason kids don't get Indy. If they only wanted wild CGI, Speed Racer would have been the biggest film of the year, not Batman.
In an odd way, I think Indy is too sophisticated for kids. For instance, I teach High School in NYC and I was talking to some kids about KOTCS, and they hated it. But they had no idea what it was about! They had never heard of Area 51. They didn't know about the Cold War, they knew nothing of Inca/Mayan lore, and they didn't understand the Indy/Marion stuff.
In a way, Indy asks you to bring some knowledge of history and legends with you into the theater, while SW just asks you to bring your imagination.
Another point...I don't think that movie popularity and big toy sales go hand in hand. A few years back Hulk Hands were a huge seller, and the movie tanked. And I bet that SW made more money on toys in August that The Dark Knight makes all year.