You can never be too safe when it comes to your kids. Riding motocross is no exception. You can look it from two different aspect. Safety gear is a priority and your choice of bike for your child and making sure they know how to handle that bike is the other main aspect of having good safety practices for your child.
First of all, make sure that your child is ready to ride. A child of 9 may possess far more responsibility in riding than another 12 year old. No one knows your child better than you, look objectively at your own child. No matter how badly they want to ride, ask yourself if your child can be responsible to look out for others when riding, pay attention and will practice good safety lessons. If your child is ready, you're ready to go.
Make sure your child has a bike that fits their body. Don't get a bike for your child to "grow" into. This is a way injuries happen. Get a bike for your child at this time so proper riding techniques and safe practices when riding can be learned the correct way. Enroll your child in lessons. Let a pro teach your child proper techniques and children generally listen to riding instructors better than their own parents studies have shown.
Next, get the correct safety gear for your child. Get a good fitting helmet, goggles and full racing outfits. Don't skimp. Don't use a previously owned helmet. Helmets are good for one impact only and you wouldn't want to take the chance with your child's health whether or not that helmet has been used.
Last but not least,use common sense. Never let your child ride alone or unsupervised. Never let them ride when they are tired or taking any type of cold medication. Make sure your child is responsible to read signs for right of way on the track. With a little preparation,you can make sure your child has a great and safe time riding motocross!