I would say you have a responsibility as a caregiver to call protective services. So many ppl don't want to get involved but but of you don't seek help for them who will?
the mom know already but I think she is in denial stage frankly speaking thats what I am thinking my someone manipulate them or play their private part before already when they go in their dads place every week end. Or they avaible to watch EX rated movie in the house of their dad
I don't like what is implied by their behavior, the older boy is acting like he has had sex or has seen someone having it or has had someone fondle him. Is there any way to talk to his mother about what you are seeing?
my relationship to them is just a nanny the kids parents are divorce. I observe them every second they want to kiss their mom when she is around
What is your relationship to the parents of the kids? Are you simply an employee, or are you taking care of these kids for relatives, or what? It sounds like they have been way inappropriately introduced to some sexual activities, whether they use them for aggression or because someone has been doing this to them for sexual reasons. I would be tempted to report what you have seen to Children's Services, especially if you feel there are other questions about their care and the parenting they have been receiving.