The Chelkas Yoav (vol.1:1) says that when a katan (minor) eats a forbidden food his enjoyment of the food is considered an aveira. The reason that he is not punished is since he is unresponsible for his actions he is considered an oness (acting unwillingly).
He then asks if so why does the Gemara need to use a is a special pasuk "Lo Saachilum" to teach that an adult may not feed a minor non-kosher food, why wouldn't this be included in the general prohibition of lifnei iver lo titen michshol (causing or being an accessory to someone else's sinning)?
He answers that although when the katan eats the food and enjoys it it is considered a maaseh aveira (act of sin), it is not considered a michshol (stumble), since (as mentioned above) he is considered an oness.
We can clearly deduce from this that, in his opinion, when someone causes someone to commit an aveira through oness, there is no aveira of lifnei iver.
In the same sefer (קונטרס בעניני אונס ענף ג), in regard to מכניס עצמו לאונס (someone who brings himself into a situation where he does an aveira through oness), the author writes:
מצד הסברא נראה לי ברור דפטור כיון דמ"מ בשעה שעובר אנוס הוא... אף דבודאי איסור דאורייתא עשה במה שהכניס עצמו לאונס משום לפני עור לא תתן מכשול, דמה לי מכשיל אחרים או שמכשיל את עצמו, אבל מ"מ בגוף העבירה אנוס הוא.
When someone causes himself into a situation where he does an aveira through oness, although he definitely transgresses the aveira of lifnei iver lo titen michshol, for there is no difference between when one causes another to stumble, or if he causes himself to stumble, nevertheless he is not punished with the punishment of the aveira itself, since after all as far as the aveira is concerned he is an oness.
This shows that even when the sinner is considered an oness and therefore not punished for his aveira, the one who caused him to do it is nevertheless guilty of lifnei iver.
This seems to be in contradiction to that which he said by a katan. How can we resolve it?