The boy sat in the chair was not accepted. How does one sit "on" a chair? In this context to sit "on" a chair would be "on top of" a chair and it would rarely be used if at all. How would one say "the boy sat in the chair" in Danish?
I think it depends on the chair... I would sit "in" an armchair, but I would sit "on" a dining chair. I don't know if there are any hard and fast rules, but I suspect it comes down to the shape of the chair in question.
In any case, I think both "on" and "in" should be accepted, unless there is a hard distinction in Danish.