The Brazilian flag is nominally "quite" simple: Green with a yellow/gold parallelogram/rhombus with a blue circle ... then there is the white band and stars. Absolute nightmare to classify.
I thought the UK flag was a bit of a challenge and I remember an art class 50 odd years ago, trying to paint the bloody thing. Little did I know at the time about how Aussies and Kiwi worriers and co would fare! Won't someone think of Hawaiians: their state flag is not something you depict for a quick laugh!
I suppose the US flag is arguably the hardest to depict: Those stars would take ages to draw/paint/dribble. I imagine kids get a box of stars to stick on their depictions in art classes in the US.
Often, when the Flag's depicted in kids' art, paints or paint pens are used, since 'negative space' stars would take forever to do with a blue marker or pencil but is quicker to paint on top of blue. And there's a great deal of artistic license used in terms of their size and color. Or on a small flag, just a lot of tiny white dots!