Childs ankle - accessory ossification affecting the distal fibula but given swelling could be partial evulsion superimposed on accessory? fracture?

Could be any: Trauma could cause an adjacent sprain, fracture or other injury adjacent to the accessory center. A delayed x-ray looking for signs of healing might be of benefit.

See doctor: Depending on your child's age a pediatric orthopaedic surgeon or orthopaedic foot and ankle specialist would be good options. Without any specific trauma it's unlikely to be a "fracture." it is possible for accessory ossification centers to become symptommatic.

It is difficult to : Differentiate the above as it could be any of them. Since, there is swelling though, i would treat the condition. I would place the child in a cam walker boot, and ice the ankle when given a chance.