Draw 5 parallel lines on the ground in chalk (or lay out five ropes)

This is called a staff

Add a picture of a Treble Clef and or Base Clef on the left.  You can start with one and switch or just choose the treble clef

Label each line and space with the correct notes - you can remove these later when the Cubs have learnt them and you can test their memory

Cubs to come up one at a time and throw a bean bag or stone onto the staff.  They need to give the note from memory once they have had a chance.  If you have an app or musical instrument available, you can play the note they landed on and once they have all had a chance, you have a "composition" made by the Cubs