Which preposition, for or to, is correct in below sentence?
It is less stressful [for/to] a child than an adult to learn a foreign language.
The preposition for introduces the person who feels stressful, regardless of the verb form used:
You can omit the subject of the stress:
NGrams shows that to is also used but for is the preferred preposition. It also shows that stress was popularised in the late 20th century.
"For" is the right choice, but it would be even better as:
It is less stressful for a child to learn a foreign language than for an adult.