Cryptic

   They were about 6 and 7 years old, waiting for the schoolbus one frosty morning. The bus driver wanted them outside, and near the end of the driveway when he pulled up in front of our house. So I would watch them from  the living room window. I saw the two of them writing on the frosted windows of my car, in busy collaboration with whatever they were scratching into the layer of frost. When I went out later, I read what was still visible on the car window:  "Kill Popeye. Go for Bluto." 
    When they came home from school, I asked them what the message  meant, and where they had found  it or heard of it. Neither of them could or would answer the question. They had no memory of it.