Prior to refrigeration and reliable sources of clean water, beer and/or wine was the beverage of choice, breakfast, lunch, and dinner, for everyone regardless of age, in most cultures. It was frequently low in alcohol or diluted.
Now kids can buy their own, and not steal their dad's beer.
But really, I don't see the harm when it's a Child's beer, meant for children. Meaning small amounts. Besides, kids'll drink someday, may as well help them build a metabolism.