From a club building and fitting perspective, we consider the age of a junior golfer to be between 13-18. This is when they presumably reach full physical maturity. Kids would be considered to be between the ages of 6-11. There are significant differences in strength and muscle development between these two age groups.
Therefore, you need to consider a custom kids set if the child falls within the lower age range. Unless a child is unusually large for his or her age – junior clubs will likely be a stretch and cause the golfer to develop poor swing mechanics. Once cemented into their swing, poor mechanics can be a difficult task to correct as the child grows and continues to play golf. Moving up to junior clubs will not necessarily correct the issue.
You are much better off to start with a set designed for younger kids and then move up to a junior set as the child grows physically into it. Kids clubs will typically be shorter and lighter than junior clubs.