If we treat my child's ankle sprain will it be gone, or do i need to keep worrying?

Second opinion: Children don't usually sprain thie ankles. A sprain is a ligament injury and in kids the ligaments generally are stronger than their growth plates thus the injury usually follows the path of least resistance and you have a growth plate fracture rather than a sprain. Maybe get another opinion if you think your child has a sprain.

Depends: Some severe sprains take over a year to resolve, usually kids heal faster though. If an injury persists longer than a few weeks in your child, consider a subtle fracture-- try looking up salter harris type 1 injuries for more info. Call a doctor if this matches their symptoms. If it is a sprain, it should fully resolve, but it's still good to have it checked for concomitant injury.