My 5yr old complains his legs hurt, to the point he cries. i had the same thing, but never knew what it was. what's causing it?

Growing pains?: "growing pains" are a common cause of leg pain in children this age. It generally occurs in the evenings or late afternoons, perhaps after children have been more active. These types of pains are usually seen in both legs, whereas concerning leg pain is always on the same side, causes limp, or comes with a fever or swelling. See your doctor if you have further concerns.

Make Sure...: ...That are no muscular or neurological issues. "growing pains" do occur, but i hesitate to firmly make that diagnosis in a 5 year unless I am sure there are no other issues. Inflexibilty from birth, hypermobility syndrome, muscle disease such as tightness or spasticity, a tethered spinal cord are examples of conditions that cause pain. Check with the doctor before writing it off to growing pains.