Not necessarily. They might not understand the sexual nature of what their doing, which means they just find your reaction entertaining in some way. This is especially true for girls as young as the one in the pic.
Kids are a lot more direct about their flirting anyway. For example, they'll bounce on your knee, or make excuses to hug you, or kiss you, or whatever.
Keep in mind that every kid is different, and you should just based on your experience and understanding of the boy or girl who is behaving like this. Trust your judgement, and pay attention to their choice of words and actions.