What's really hard, together with the quantity of them is that they're white! I haven't actually seen many star stickers used, but agree that's a great idea.

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!