Video on the process here. 

Classes in PK - 3 would be assigned an art task such as:

Paint leaves, stamp leaves
Create flower petals
Concentric centers
Cut leaves and flowers
Assembly and details

Perhaps each classroom can create it's own version?

This might be the loftiest of the three ideas.

I'm not sure if there is a budget but this would look great on canvases. A cheaper option would be do this on foam core board.