Dirt Bikes are pretty safe for kids as long as you teach the kid the proper safe riding instructions and have the kid wear the proper gear and helmet.
Make sure you have the kid ride the proper size of dirt bike with just enough power for the kids age and size.
When you go to a dealership for a dirt bike for the kid they will know what size of dirt bike is proper for your kids age and size.
Teach the kid how to ride the dirt bike safely and only use the dirt bike on the proper dirt bike tracks etc and never on the road and your kid should be very safe on the dirt bike.