At that age to be displaying this behavior is a red flag that this boy has been sexually molested and he should be seen by a professional to uncover the event. That is why he has an overt obsession with showing his penis to another person. The fact that he is intent on showing it a girl indicates that the molestation took place by a man. It is a serious issue that will haunt the boy his whole life if he does not receive proper attention for it now. If left untreated it will end with him being a sexual predator.
Thank you all for your input. I have the whole bunch living with me now due to unforseen financial difficulties. I'm keeping an eye on things and I know my daughter is vigilant. We have also educated my granddaughter that she doesn't have to look, she should leave the room and she should tell us if anyone exposes themselves or touches her in a way that makes her uncomfortable. (hopefully this kind of thing will always make her uncomfortable). She has a healthy self esteem, so hopefully her laughter was enough to deter any further expos.
I agree with margypops. At 8 this kid should know better and you need to be aware that kids are engaging in sex between 8 & 10. Shocking, yes but that's the statistics. We came from a different era, no longer are children so innocent at 8 as we were. The father should have been appalled at his son's actions, I would be. It's very disrespectful at the very least, and I would worry when these two are alone. This will escalate, so your daughter needs to address this now with the father and talk to both children. If the father still finds it acceptable, I would kick him to the curb and keep my daughter safe.
At 8, kids are more into potty humor than sexual acting out, and despite the frontal nature of his waggling, it sounds more like he thinks he's mooning than like he's exposing himself for a sexual thrill. I rather disagree that it sounds like a response to having been molested or anything overtly sexual. I agree with you that if it was an older boy, it would be a threatening thing with sexual overtones, but 8 years old ... well, all I can say is that some of the rowdier boys I knew at 8 would probably have done things like that (I remember some of them playing with poop in a park. They would have thought mooning was super-hilarious, if they had been bright enough to come up with the idea).
What your sister can do is talk to the boy in front of his dad and ask the 8-year-old not to do this in front of her child. She doesn't have to fight with her boyfriend over the whole is-this-normal-or-not issue. She can just address it on the "I'm a mom and there are rules of social decorum that I want you to obey around my daughter" level. Some judgments aren't worth arguing over, and she can't win anyway if the dad is defending his son. She definitely can ask the child to behave in a more socially acceptable way.
You are right to be concerned,this is totally inappropiate behavior, it could be the 8 year old has learned it. Your Daughter needs to make sure her daughter is not alone with the boy and even tell the boy herself to stop doing it.Its up to your daughter to take control and protect her child ..Who knows what else this boy will do especially as the father thinks its 'normal' maybe your daughter should fine a new bF who does'nt consider lewd behavior to a young girl , okay ...Good luck its good you are concerned .....