I get my kids involved as much as they're willing. I see it as a way to introduce measurement, attention to detail/process, chemistry, biology, as well as removing the mystery around beer and alcohol. My oldest (almost 5) starts most every Saturday by asking if we can brew. Her favorite part is right at the beginning, helping to measure the grain and she always gets a little cup of 2 row to munch on. We discuss the process, the mash, the ingredients, etc. Afterwards, she spends days telling mommy about the whole process. She got really excited a few weeks back when we did our first ever batch of root beer.
The almost three year old and the 1 year old obviously don't yet show any interest, but on many occasions I have them around while the process is going on.
A photo from last winter of one of our brew days.