It could have been done as a sick form of manipulation "see it feels good so you like it and you want it" when really the child may not like or want it at all but the nerves in their body are still sending pleasure signals to the brain. Also, once a child is sexual with themselves or others (even if forcefully) they often become more sexual themselves seeking out such pleasurable stimulation. So the parents could be manipulating the child in that way as well. Never really giving the child a choice, but making the child believe they want it only because it feels good.

Which is fucked up, and of course I am just theorizing. I have no idea what the parents did and how the kids felt.