I hate seeing this place called a "belt factory", but only because I did martial arts with people associated with this place yrs ago and though I don't know who is in charge these days I can tell you the idea is not to be a belt factory. There are great instructors and just because they tell you they are ready for the next belt doesn't mean you have to let your kid do it if you want to slow them down.
With proper instructors who care about the kids and not the money I think martial arts is great for kids!! I recommend trying out a couple different places until you find what you're looking for for your child. If you just wanna give away money and let your kid THINK he's the next Bruce Lee then go to a "belt factory"!!
They usually have a cheap 1 month "trial", so try it out and see what you think. Then you can decide which place is good for your kid long term.