It will depend on the age of your child, and how large the flag needs to be, but here are a few suggestions:
-Have your child cut blue and white strips of paper, and glue them to another sheet in the form of the Greek flag (Google search for an image if you are unsure what it looks like).
- Help your child fingerpaint the blue lines of the Greek flag onto a white paper
- Help her sew blue fabric (or felt) strips to a white background to create a fabric flag
- for a larger flag, cut out LOTS of blue hand prints and white hand prints, then arrange those to look like the Greek flag

Have fun!