I have bought 3 boxes of this product and kept every card I have traded for several hundred of these cards as well and never pulled or seen one but, I have heard of them. What I remember from years ago they were known as hidden messages designed for kids
like the one you have there is another card that says
BE COOL STAY IN SCHOOL
now I am not positive but they were supposed to be on random cards so not every card in the set would have the hidden message and there were 5. If I'm not mistaken. I would be stoked if I ever get one of these