Is it normal for little kids?

My 9 y.o. daughter has two best friends:boy same age,and girl one year older,i noticed that generally they games consist of girls rides on boy as on horsie,,sometimes even both together. Actually i wouldn't worry too much about it, but sometimes i think girls take him as their real horsie, like when sometimes i called them while they play this game, the girl, who rode on him at that moment, just ride on him, instead of walking.