My 5 year old son is a not often sleepwalker/sleeptalker but tonightbi noticed his legs shaking while asleep. what could this be?

May be normal...: May be normal to have many types of movements when falling asleep, during sleep, and when waking up. If a child is otherwise a happy, normal kid, then parents can just make a video of the movements to show the doctor. Parents can add details such as: when it happens, how long it lasts, what foods or meds taken, etc... During some episodes on video, wake him up while showing his response on video.