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It is very difficult to ascertain what the swelling is. A varicose vein is a dilated tortuous vein with reverse flow. What you describe can be a dilated vein but to ascertain whether it is a varicose vein is difficult to say. A varicose vein can cause pain in the legs. But at this age there are many more causes which can lead to leg pain like anemia
etc. I suggest you consult a Pediatric Surgeon for further evaluation. Till then keep his legs elevated especially in the night. Give him some pillows below his leg. If its varicose veins
, it will help. Some tight stockings will also help but i guess you should get a diagnosis first.
Hope that helps
God bless you