It is generally accepted that kids learn most sports faster than adults (once their coordination has matured), but tend to find it harder to maintain focus on one activity.
With that it mind, you will often find that children will benefit greatly from some early coaching, since they will pick up the basic stroke techniques very quickly. However, they may then find it difficult to keep motivated in long-term lessons, due to the wide variety of other sports and activities that are competing for their attention.
So I would recommend that if your child has shown an interest in ping-pong, you should get around 6-12 lessons from an experienced coach. Your child will then be able to learn the basics of modern technique, and can continue with the lessons if he or she wants to progress further. If your child decides that table tennis will be an occasional activity, at least they will have a good grasp of the basics to fall back on if they ever decide to take up the sport again in later years.
It's a little bit like riding a bike, if you learn as a kid you can come back 20 years later, hop on a bike and away you go. Trying to learn to ride a bicycle as an adult is a whole lot tougher!
