Parents dont want to help, How can I?

Hello, I know this is a lot to read but I really need to tell someone! And I would really appreciate any feedback!

I am a full-time nanny for a family in which the parents are rarely home...

I nanny for 3 children, a 5 year old boy and his 2 older sisters...On occasion I nanny my regular 3 and their 3 cousins, they are the husbands brothers children...

Anyways, their cousins are 2 younger girls and a 7 year old son, we'll name my 5 year old that I nanny full-time Rob and the 7 year old cousin Mark.

Last weekend, I did an overnight sit with all 6 children, Rob (5 yr old) and Mark (7 yr old) were sleeping in bunk beds in one room and the 4 other girls split up in other rooms.

Well, I snuck upstairs to check on the boys because I have had problems with them talking and not going to bed before, so I went to the door and heard whispering I put my ear against the door to listen closer...

To my horror I heard Mark all the sudden say "Ok, I want you to lick my balls" (I was in shock, and honestly couldn't move)
Rob replied saying almost jokingly "Ok, you can lick my butt"
Mark then says "Ok, Rob, you want me to lick your butt right?, Rob replies "Yes" and Mark said "Well if I lick your butt then im going to stick my weiner in your butt"

Immediately after hearing this I busted open the door and Rob jumped down from Marks top bunk and hurried under the covers and I noticed his pants were off! I then said what is going on and Rob said he was hot and thats why he took his clothes off, Mark was pretending to be asleep and I did not want to make a big deal of it and I honestly really had no clue what to do...I brought Rob downstairs with me and he fell asleep within minutes...

The next day I told the parents I nanny for and at first they were shocked and then the mother said well I dont want to tell Marks parents today because today is Marks moms birthday and I dont want to ruin it, I said that was fine and that I would tell her the next morning,,,The mothers of Rob and Mark are very rich and spoiled and I often notice a jeliousy between the 2 with their children,,,Robs mother had said many times "How am I going to tell Marks mom, she is going to get defensive" and she kept saying how they are together all the time and it would be akward for them after telling...

Well, the next morning the mother I nanny for called me and said "My husband and I decided we do not want to tell Marks parents about this, as long as we know we will keep Rob from being alone with Mark, but we think its not as bad as it sounds" (Almost making me feel as if I was blowing things way out of proportion!) and I really believe the only reason they are not telling Marks parents is because they do not want to make the relationship with the parents weird or uncomfortable for them!!

I couldn't believe it! I mean this is their 5 year old son who was being talked to like this by his 7 year old cousin! The two families are busy making multi-million dollar plans to build homes right next door to each other! Mark and Rob will be living right next door in a matter of a year and the parents think its no big deal!

The parents said they talked to Rob and told him its not appropriate to be touched like that and Rob said "Well mom Mark is my best friend" and she said her and her husband told him over again that it was not allowed and to tell a grown up if it happens again!

Meanwhile, before this incident happened I had talked to the parents about Robs behavior (since the parents are never there to see it) I told them how all the sudden Rob would never like to be alone, he was scared someone was going to get him, he would make me hold his hand and walk him where ever he wanted to go thru the house, he also began being very hyper and not listening or paying attention to me...He had also had 2 letters sent home in a month for bad behavior, hitting and lashing out and hes only in kindergarten! Also, I was very worried because he began pooping in his pants...I thought maybe this was all to fight for attention from his parents but now I wonder if any of these issues are in relation to possible sexual abuse?

I do not believe this behavior is normal, The specific acts that Mark stated makes me worry...I believe it is Mark that was influencing Rob because I am with Rob 247 and he has never once spoken about his penis or made any reference to it..He also was giggling and jokingly talking back to Mark as he was being asked to do things that night.

Do you think Mark has been or is being molested by someone else?

Why would Mark want to do these things with his cousin, and another boy for that reason?

Do you think the actions of Robs behavior could be from molestation?

What can I do to let Rob know without making him scared or upset that it is not ok to be touched in those ways?

What possible future problems could Rob have from his cousin doing this to him?

I told the parents I nanny for that I refuse to ever nanny the other 3 children again, and I feel I have done my part by telling the parents I nanny for, but I want to tell the other parents also, Although I feel like I cant...I love my 3 children I nanny for, I cant imagine them out of my life and honestly I dont trust them to be with anyone else, I know even though their parents are basically non-existant that I am there for them 100%...

This is such a terrible situation, Any advice or suggestions you have would be great! Thank you for your time!!!