15month granddaughter suddenly won't walk! has a small blister on foot from sandals. eating, drinking, good mood. on iron. should i worry/visit er?

Yes: Pain is the only reason a previously walking toddler will not walk spontaneously. There possible is a chance of a joint inflamation or infection or a injured bone. A visit to the er or a pediatrician asap is appropriate to make sure there is no problem\.

Stop the pain: Shes probably not walking due to pain from the blister poke a hole in the blister drain it then apply betadine and dont let her wear those sandals again.

Why not : Visit the pediatrician or podiatrist...Why run to the er? You yourself stated there is a blister of some sort...Maybe that is the issue.

She needs to be : Checked by her pediatrician & referred to a pediatric orthopedist if she has transient synovitis of the hip, a self-limiting condition with inflammation of the inner lining (the synovium) of the capsule of the hip joint. It may be associated with a viral infection or minor injury. If she doesn't have full range of motion on exam, an ultrasound of the hip joint may show ⬆'ed fluid.