Millet is fine for hens and as a very occasional treat for chicks under 6 weeks old since (for optimal health) their diets needs to be at least 19% protein. BTW The available protein in Millet will be increased 2-3% if you soak it for a few hours before giving it to your chickens.
I feed my hens/chicks a mix of 1/3 raw sunflower seeds.1/3 millet, 1/3 rolled oats (with a handful of grit) as a treat instead of bagged "scratch". I buy the ingredients (cheap) from the bulk bins at my grocer.