Just saying clean up, is very vague. So is "a tidy room".  The following is magic in my Pre-Nursery:

"Okay kids, I need 20 lego pieces in this box!!"  As they put the legos in, you call out their names and say what number piece they are.  This gives the children specific instructions, and that child gets his attention, everyone else hears and wants to join; and you have a goal (20 pieces) so it's fun:  "Number 1 is from Chaim, #2 came in the box from yehoshua".