Pushing something on your child is never a positive thing. There is a fine line between helping a child who is already interested in riding motorcross and pushing a child with no interest in the sport into doing it because it's something other brothers and sisters or the parents are interested in.
If your child is already interested in motorcross, let them hone their interest by looking in magazines, reading about Supercross heroes such as Chad Reed and Jeremy McGrath. Your child can learn alot about good sportsmanship and being a good winner and a graceful loser through competing.
Take your child to a local motorcross track and let them watch the riders. If that doesn't peak their interest in the sport, nothing will. Motorcross is very exciting and most gets really enjoy it.
If your child isn't interested in motorcross, he or she may not pay attention to the safety rules or even be interested in learning proper safety and motorcross bike handling techniques, this is dangerous not only for your child, but other motorcross riders around your child.
Get your child a used bike in good working order and let them ride the motorcross bike and get a feel for it. Some kids are really interested in the sport of motorcross and just need a little nudging in that direction.
Also, does your child know and understand the proper use of all the safety gear? That is so important. You're not going to be leaving your child riding a motorcross bike unattended but you're not going to be on the back of the motorcross bike either. Your child has to understand the basic importance of the helmet, goggles, pads and other safety gear. Motorcross is a sport the whole family can enjoy and something you can participate in with your child.