The only thing that makes Brazilian Jiu Jitsu "gentle" is the will of the practitioner. Jiu Jitsu teaches the use of leverage which if not held back can easily break limbs of much larger attackers.
John Valenty's daughters Sophia Valenty and Fiona Valenty have been training since they were about 10 years old. Now that they are both 105 pound teens, they are quite confident and formidable individuals, beautiful and straight A (4.0) students too. Don't mess with them, you won't even see it coming :-)
Bottom line is that Brazilian Jiu Jitsu is the right base to learn before any other martial art or fighting skill to become more confident and for kids, they just cannot be bullied with these skills. I highly recommend it for children from about age eight and up.