I think whether its crime or not is a bit of a red herring. It certainly can be abusive whether or not that meets the threshold for the specific crime
There are plenty of families we work with whereby the child is exhibiting sexualised behaviour toward themselves or other children/adults, at this point there is often exploration of whether the child has witnessed sexual behaviour at home/been abused. Quite often the child has been exposed to the adults having sex,, not caring if the child is in the room, or seeing sexual behaviour online/on screen/hearing language about it. Its normal for people to wake up during sleep, or at least absorb what is going on around them while asleep so the child is being exposed.
So whether or not a prosecution comes from that (extremely rare), it is abusive.