With a 4 year old? That's a brave thing to do - I hope you are good friends with your cleaning equipment *lol*
Even the most talented kid that age will have problems mastering any technique, I fear, but I know someone who gives classes for kids 6+, and I know that she works a lot with rather stiff icing, food color pens, and sprinkles.
Stiff icing has the advantage that it is easier to handle and less likely to go everywhere but on the cookie. Techniques that go with it are easy track-and-trace patterns (which you can draw on a blank cookie in advance), e.g. a Christmas tree with dots as baubles. You might even give brush embroidery a try.
Food color pens are probably the easiest and least likely to cause a complete mess. You can ice some cookies in plain white in advance, stamp or draw simple black outlines with a food color pen, and let the kids paint them just like a coloring book. This is something I do for my nephews, 4 & 6, pretty regularly - and they just love it!
Sprinkles are a nice way to give the kids a sense of achievement. They can go wild with the stuff and stick it to their stiff icing designs, or you can help them to flood a cookie and then dab them into their favorite sprinkle. But please note that you will have sprinkles all over the place, even in rooms the girls have never been to, for weeks. And this is also a personal experience. My aunt is still complaining about the mess my cousin and I made 35 years ago
I hope you will have fun with your friend and her daughters!