My daughter is 5 and has been taking karate since she was 3.5. She absolutely loves it! It has definitely thelped her self-control, respect for self and others, discipline and focus.
At this age, and at her school, the focus is certainly more on self respect and having fun than on specific techiques, but it is a great start.
Most reputable schools will let you try for a full month for free. Don't hesitate to shop around: each school has a different philosophy and teaching style, and it may take a little time to find something that your child likes.