My opinion is until you are 14/15 mostly work on skill, speed and do BWEs like push ups and sit ups. You can lift at 12 and it won't kill you but it is a waste of time as the hormones have not kicked in yet and a few push ups will give you basically the same result. At 14-15 however weights will make you stronger really fast so this is the best time to start real weights.
It is really important to work speed as kid though and specifically stride frequency. You can develope stride length later but if you haven't developed the motor ability to turn over the legs fast as a kid you wont get fast no matter how strong you are.