Is the death of a son considered a shortcoming on the father's part, necessitating expiation or blood money, especially since the son fell and died after the father placed him behind a curtain, thinking it was a wall?

You must expiate by fasting for two consecutive months and pay blood money to the child's heirs, because he died due to your actions, and you are not sinful since you did not intend to kill. Be patient and seek your reward from Allah, for you will be rewarded for your patience. Ask Allah to compensate you with something better.