whats popular here is like Jimbo said, more fluff than tuff for kids, so in that light, if your goal as a parent is to give your kid a nice workout and “fellowship” after school or for daycare, that works. It doesnt help the quality of the young martial artist of the future however, by selling black belts or allowing kids to pass tests just for good sport. There should be a division of curriculum between “recreational” students and students of “the discipline” , whatever it may be.
In relation you can look at it like an art class. Most kids go to splash pant around and make a colorful picture, however, that can also be a place where a kid can begin to take their painting seriously.
my opinion is more slated towards the old African and Chinese ways of rites of passage (types vary depending on where you are) starting in early childhood. there is no politically correct way to do it on those terms. Its brutal to some, yet the method works.
Shaolin starts kids training at 4 for various reasons. One of the main ones is the bodys is supple and shape able at that age. Kung Fu is a long hard journey and if a kid wants a head start, the best way is to stay loose, build that early stabilizing balance and strength that can be easily cultivated in to speed and power and explosive agility as an adolescence and mastery as a man. Any of you can probably testify to the difficulty of flexibility as an adult, ESPECIALLY if you didn’t cultivate it in early life and maintain it.
Another reason is discipline. The honest truth is, a kid who went thru vigorous training early will be better tempered emotionally and mentally, and also develop the ability to start something, be patient and work at it, and attain efficiency. These are priceless qualities for a kid to have going into teen years.
like the RZA says “WU TANG IS FOR THE BABIES!!” lol and I echo that. teach them the ways early, and save them the trouble later, from the physical discipline to the meditation, and even philosophy of martial arts to the best of their comprehension.
There are obviously wrong and right ways to teach a child martial arts, so I’ll spare listing them. in general however, I think between the ages of 4 to 7 are the best times to start seeding youth with good, pro active habits that cultivate vital qualities like patience, focus, esteem, understanding, poise and discipline. Martial Arts covers all of that.
Shaolin does it best in my opinion, and always has the troops to show and prove the work! 
Amituofo