My 3 year old stepped on something and now there's a red line streaking up his ankle from the wound. What is this?

See ur pediatrician. Could be a sign of infection, should be taken to his pediatrician asap.
Be seen. It certainly sounds like an infection. He needs to be seen immediately.
Lymphangiitis or. "blood poisoning". The infection, usually caused by streptococcus by may also be staphylococcus, has gotten into the lymphatic system & is spreading. Take child to fp or er asap for treatment to prevent further complications. Make certain child is up-to-date on tetanus immunizations.