Being a martial arts instructor, I get this question all the time. Unfortunately there are very few school out there that will give you a one hundred percent honest answer on this subject. The truth is, yes martial arts can aid in teaching children focus and discipline, but not without the support of the parent at home. Most martial arts classes are held two to three times a week for maybe an hour each. This alone is not enough time to teach any child to be focused and disciplined at home and in school. It takes constant work from the parents to properly teach focus and discpline.
Martial arts aids in this in two ways: One, by reinforcing the childs learning by showing the child that focus and discipline are also required in hobbies, games, and other areas of life that the child may not think focus or discipline applies. Two, teaching the child not only to focus and be disciplined, but also how to be focused and disciplined.
So all in all martial arts does help children immensely, but it will have absolutely no effect if the childs learning does not continue outside of the training hall. Other things to keep in mind are that not all martial arts, or even specific school locations, are geared toward teaching focus and discpline. Many schools only teach self defense, some only teach sport martial arts.
The schools you want to look for are any schools that advertise "traditional" arts or specifically state that they teach focus, discipline, and/or self control. Try to find school that have well structured classes, and schools that have strict rules. The best schools are those that are relaxed and patient when children are learning something new, but are also strict and agressive when having children practice things they already know. Another good thing to look for are schools that require forms to be signed by parents and teachers upon testing to ensure that the child is behaving at home and school.
I hope this helps not only answer your question, but also help you find what you are looking for if you want to put your child into the martial arts for focus and discipline reasons. Just be warey that in order for the child to learn and apply these traits, the child must be learning to use them at home and in school as well. Good luck!