well quite a question there Gumby
Some kids (especially the school aged kids who have had the good-touch-bad touch presentations at school or other kids who have been taught about this actually do know the difference and this is something bad (even though it feels good)
The problem lies with younger kids and those who have not been instructed in this matter.
Some children are so yound that they really don't know this is something "bad". The person abusing them is someone they trust and very often someone who is not hurting them physically. The touch they recieve actiually does feel good (although some studies say not the same as later sexuality). Most often these young kids just think this is a secret game. And the rewards some get make the game worthwile to them. They don't understand adult sexuality so they don't know what is going on. Just like picking up your toys gets you a reward so too this game might get a reward and often an abuser will use rewards as an incentive.
But there comes a point when children realize there is something not quite right about this game. Sometimes once the threats to be quiet start, or if there is physical pain, or if the child realizes that even though they would rather do something else they still have to play the game first their awareness changes about the game. Sometimes the threats come rather early but if few people have access to a child the abuser may think she/he is safe for a while and not resort to threats to silence the victim and protect him/herself.
Sometimes the abuser isn't playing games and the child knows it. The fear comes from threats and violence and feeling trapped. Kids around the age of 6 - 7 begine to be extremely private about their bodies. So someone touching them crosses those boundaries and they are keenly aware of it. Veyr often we teach children that their private parts are "dirty". So now you have an activity which includes touching those "dirty" parts. Kids get confused easily. It feels good but it is bad. If it is bad then I should feel guilty. We also tend to blame children a lot for things. So if something bad happens it must be the child's fault. And some abusers will actually tell the child it is their fault. And if the child physically enjoys the touch then they really feel bad about liking something bad.
If the child tells someone and gets a negative reaction or is thinks he/she is being blamed (Why did you go there?" or "why didn't you tell me before?" the child's feelings of guilt and shame will escalate. All the more reason to get the child to a professional instead of an inquisition about what happened. The longer the activity goes on before the disclosure the harder it is for a child to belive they are not to blame for not telling immediately. If it started out as a game and later the child realizes that is is not a game then the child starts thinking about the question "Why didn't you tell right away?"
I might think of more later