A child may be Junior Open Water certified at 10yrs old. Various training agencies have scuba programs for those 8-12yrs, who are either too young to get certified, or just not quite ready to become certified. You can check with your local dive shop to see if these programs are available.The programs are done under the supervison of an instructor in a shallow pool. These programs can help to build confidence & the maturity needed for a child to become a successful JOWD. Whether a child should get certified at age 10yrs can be debated all day long & I'm not going to get into it. There must be a certain level of maturity reached before a child should go for certification. As Thorndowyn mentioned, breathing compressed gases at depth has risks of possible injury & even possibly death. If the basic rules of scuba diving are followed, these risks are minimal, but still present. Not trying to scare you, but just stating fact. One thing I strongly recommend, is for a parent to either go through the course with the child or for the parent to already be an experienced scuba diver. The child will need an adult diving buddy anyway, why not make it a family bonding activity? If the parent is a scuba diver, I encourage them to sit in on the classes (but not interfere), that way they know what & how the child has been taught.