Can children outgrow stiff person syndrome?

yes, but it is rare

Stiff person syndrome (SPS) is an extremely rare progressive neurological disorder characterized by persistent rigidity and spasms of certain voluntary muscles, especially those of legs and feet.

There is no cure for SPS and the long-term prognosis is variable. Many patients have a slow course of the disorder that is mostly without symptoms, punctuated by occasional episodes of stiffness.

Yes. My mother suffers from this rare disease. She takes diazepam in order to control it.

The cause of stiff person syndrome is unknown, however, researchers theorize that SPS may be an autoimmune disorder.