Jiu Jitsu is a great way to get your child involved with a growing community while also making friends with other parents if you are not training yourself. It is natural for parents to worry about enrolling their kids in jiu jitsu for fear of injury or lack of information when to do it. Some academies accept kids as young as three years old in their youngest classes. If you are interested in having your child try jiu jitsu, inquire with your local academies what their minimum age requirements are.
Although the starting competition age for IBJJF Kids' events is four years old, this does not mean your child cannot start at three years old if your academy allows it. Most classes for young kids will involve a lot of physical activity aimed to help exert their energy levels while also incorporating jiu jitsu specific exercises. If you are ever concerned about your child's safety, academies generally allow and encourage parents to stay and watch. Most importantly, you can always communicate to your professor or coach if your child has any special needs or if there are any issues.
It is hard to believe that such tiny gis exist for children this young, but training apparel is available. Check out Vulkan's line of kids' gis as styles are available for kids as small as two feet tall!