I have the kids in the same class as the adults. They have been as young as 6 years.
The youngest ones only stay for an hour or so, since they burn out and get cranky if they stay too long. As they get older, I have them work out for longer periods.
Physically, the kids are more flexible, so it’s easier for them to have nice low stances and nice high kicks. They learn the form sequences really quickly, and can be more precise than the adults. They’re not as good on applications that require detailed coordination, so something like au lou choy - yiu jahm is harder for them than kwa tung choy. Working with kids is nice… they’re less stiff and rigid compared to adults.
Kids get really excited about learning, and I like for them to have fun in class.
When doing footwork, sometimes we have races. Kids that do the footwork wrong, get told, “ah, ah, no cheating!” And when they do it correctly, I tell them, “wow.. you’re getting better!” I don’t make a big deal about who wins, but it’s more about letting them get excited about the class activities. In all our activities, if the person practices and improves he “wins”, so the point is self improvement, not necessarily beating the other person.
Sometimes they get to race the teacher who has to do backwards footwork. I’ll tell them “faster, faster.. you’re almost catching me!” They’re usually laughing themselves silly by the time we’re done. Sometimes I’ll have them do one line of kicking, and they have to race back using ground sweeps, or frog jumps, or duck walks. Then they’ll do another line of kicking and another race back to the begining. I’ll say, “hmmm.. what should we do… frog jumps??” And the kids will bounce up and down… one picking sweeps, and the other picking duck walks. So of course, we do 2 lines… one line all sweeps, and the other line all duck walks 
I teach them theory and applications at the same time. Pokemon has got to be one of the best things that has happened for kungfu training. The kids are already programmed to think in terms of what defeats what. So when I ask them, “when I do this, what could you do?” Each kid/adult can have a different answer, and I like to ask each person. Then I"ll answer, “the one I was thinking of was this…” I’ll pick from the other answers for variations if suitable.
Sometimes I’ll ask each of them to think of a single attack. Then I"ll ask them to show the class. And I’ll ask them to tell the class how it’s used, and what is important to remember about it. After each kid has his turn, I"ll pick some of the motions, and make a combination attack from it. Then we all drill that as one of our applications for that day.
I have the kids start sparring when they are about 7 years old. No body conditioning until they’re in their teens and their bones get harder.
That’s my rambling for now 
N.