When your opponent attacks Dreamsprite it's the intended attack target at first, then you use its effect to switch the attack to another one of your monsters, if available, and that monster is now the attack target. This is true even is Dreamsprite is attacked while face-down.
First ruling for Dreamsprite -
"¢ The effect of "Dreamsprite" is not optional so if you have at least 1 other monster on the field when "Dreamsprite" is attacked, the attack target is changed to your other monster. This is also true if "Dreamsprite" is attacked when face-down.