There is no single best martial art, but there is a best fit. The right choice depends on your child's temperament, your goals, and above all the quality of the instructor.
Karate suits children who benefit from structure. The syllabus is fixed, progress is clearly defined, and the emphasis on etiquette shapes behaviour as much as technique.
Judo is grappling-focused and excellent for physical confidence, though it involves more direct body contact. Taekwondo emphasises high kicks and flexibility. Kickboxing is fitness-oriented but usually lacks a traditional value framework.
Shito-Ryu specifically is known for having the largest kata syllabus of the major karate styles, blending the hard linear techniques of Shuri-te with the circular, breathing-focused methods of Naha-te. That variety keeps children engaged for years.
When you visit a dojo, watch a class before enrolling. Look at how the instructor corrects a beginner — that tells you more than any brochure.