Should an aggressive child take martial arts?
This is a complex question that runs deeper than you think. If you'll bear with me, I'll go through it in depth.
First "an aggressive child" says the child IS aggressive. This is an oppressive and limiting perspective of the child who accurately is a "child who acts or has acted aggressively". There are a great many reasons for aggressive behaviour in children, the most readily assessable are abuse of any kind, or witnessed abuse, specifically continued exposure to abuse. this can be from school, from home, from any regular contact with an abusive situation.
So, even before you take action on martial arts, CHECK your assumptions and projections. The child may very well be acting out learned behavior from an abusive experience, neglect, etc. Get to know the childs history enough to know where the aggressive action comes from.
Whether or not you can see the source of the aggression clearly, DO clear your mind of the perception that the child IS aggressive and drill it into your head that the child ACTS aggressively.
I work with children both in my home and in my work place, both teaching and coaching. It is imperative for the child that this perception shift takes place in YOU and those caregiving for the child, for the child to have the best chance at learning behaviors that are socially beneficial, respectful and healthy.
After this process, or even during, enrolling the child in something that is physically active, non abusive, and most importantly inspiring to the child can be very beneficial in channeling the childs energy. I brought my oldest to kung fu and tae kwon do, and though I believe both to be very beneficial for humanity, he was unhappy in both, to the point of feeling ill going. So we left it alone. However, we watched some parkour videos and he was hooked. He now takes Parkour classes, practices his exercises daily at home because he WANTS to and LOVES to, and his only issue with it is that he can't go to the gym more often, or go for runs in the city more often.
So get to know the child- perhaps dance is better for them, tribal, hiphop, contemporary, etc. Maybe it's parkour, or maybe a sport. Expose them to a variety of physical activities that demand skill, focus, practice and go by their passion.
Let us know how this pans out...unless of course this is a hypothetical child, in which case... :P