10 year old. Severe pain stretching from the thigh to calf without prior injury?

Unusual... If it is truly severe without any history of trauma, it is worthwhile to have the child examined by a health care professional. Are there any other associated symptoms (swelling of the extremity, for example)? Swelling with this type of pain could mean a blot clot. A careful exam and history will help get to the bottom of what is going on. I hope the child feels better soon.