What is the obligation when a child unintentionally witnesses the parents engaging in an intimate act? If required, what type and amount of compensation, Kafara or Sadqa, should be given?
Bismihi ta'ala
There is no kaffarah or anything to be paid, but there are two very important thing everyone must always remember.
1. Parents, or anyone being intimate must secure their privacy first, making sure there will be no intrusion, and have absolute certainty that nobody can or will be able to see or hear them. Even infant children should be away from their parents, and we have narrations from Ahlul Bayt (a.s.) that strongly censure parents being intimate with the presence of even a baby.
2. Whenever someone is living with other people, he/she must always remember to announce themselves, knock doors, make a noise, say "Ya Allah", to inform others of their presence, and so on. This is to avoid seeing somebody without their clothes, or in a situation that they are uncomfortable in, etc.
In the case something happens unintentionally, it should be forgotten, not mentioned, and just be stricter in observing these two points.
And Allah knows best