Do not tell children why, just direct them to choose 15 items from your junk drawer. If you don't have a junk drawer, have them choose 15 random items that will fit in a shoe box.
The challenge: to build a robot. Only one adhesive item may be used in construction -- glue, paperclips, tape, string, etc.
Super challenge: build a robot with articulated parts.
Good luck. Don't forget to send pictures.