Will one of the children of the scoffer be afflicted with mental retardation, and what should he do to be safe from God's punishment after ridiculing a person with a slight degree of mental retardation?

Mocking others is forbidden, even if not intended as gloating. And the one who gloats is exposed to punishment. And whoever gloats over another then repents, Allah will accept his repentance and the punishment will be lifted from him.