I have worked with pre school and school age kids (2 to 12 year olds).
I have had to discuss phrases and physical situations with many parents over the years. Not just with pre k, but also with some of my 5th and 6th graders. It is never easy and it won't ever be easy to discuss these things with other parents.
Some parents took it well and was equally concerned, while other parents told me to mind my own business or brushed it off as "kids will be kids".
Kids are going to be curious. They will experiement. You will see Johnny kissing Sue on the lips at age 5. You will see Johnny and Sue asking questions about their body, about babies and you will have them repeat some words they hear from their parents. (You know those commericals aired a few years ago where the kids would say something embarressing to strangers while the parent is mortified?) lol. But as someone else posted on here, there is three levels of age appropriateness. You will hear the "show me yours and I will show you mine".
I remember doing that in K school! It wasn't because of sex itself. It was because of curiousity. Why he has a penis and I don't? I even asked my brother, 2 years older than me, to see his because I wanted to figure out how to get one myself. (I thought it was unfair to urinate sitting down). It was harmless thinking, but others could of quickly jump to conclusions that could of turned my family upside down.
I had a fascination with penis's in a non sexual way. I thought they were the coolest thing. All females had vaginas. Boring. I wanted to write my name in snow is all I kept saying lol! But that was my typical curious 5 year old thinking. Luckly I had great understanding teachers and my mom sat me down to explain in detail what makes me different and me to appreciate/like what I have etc.
So these situations should always be taken with care. I think the OP is trying her best to do that and good she is trying to get feedback instead of jumping the gun on it.
When in doubt, talk to a teacher or talk to the doctor. Ask these questions. "Is this normal of this child's age. Given her home background from your view point? Is there something I can do or say? Will this affect my child and how do I explain to my child?".
I have had true sexual abuse cases and I have had just some overly educated kids. One in particular was 8. She knew of every drug (illegal),
"boyfriends" issues between her mom, sister and aunt (abuse, cheating etc) and this girl was very educated on sex (positions to preventives: condoms etc). I feared for this girl. She knew more than I knew in high school.
I had to be very careful with that one as it also had race into it. So I let my boss handle it as I am very uneasy about handling issues that include race or traditions as that is when heated debates arise. (little girl answered my question of how come she knew so much, etc. She explained her family is black and intermix with mexicans on another side. So, it is tradition to know and do these things by 13. Then went on how her dad is in jail, how her mom thinks it is good she is exposed to the "real world" and accepting her family for who they are etc).
I have had 5th graders in the office confiding that they might be pregnant to the nurse. Sadly, I only remember one actually being pregnant. No abuse at home (good Christian family), she just wanted to impress a guy she had a crush on (6th grader). He came from a family...lets say his father is in prison for beating/raping his mom infront of him and he thinks the world of his dad. My manager was pushing for therapy as to our surprise, he wasn't recieving any.
Another 5th grader was being explained the birds and bee's in detail (her mom opt out on sex ed in 5th grade) as the mom was expecting. The girl thought if her mom is expecting, she might be too as they got their period about the same time. (to say the little details I knew on this situation). Some parents are not able to explain clearly or have the ability to.
With this 5 year old girl, the possibilities are out there on what could exactly be going on. Innocent or could very well be a red flag something inappropriate is going on.
It does matter their thought process, but it shouldn't be the deciding factor in whether you discuss this with her parents or not.
Every child is being raised differently. Some are raised with parental growth development talks while others base their teachings off of radio, tv or peers.
This girl is set up for some sexual peer pressure and future sexual situations. You do have a right to be concerned for this little girl. You are a responsible loving parent who has a gut feeling something isn't right. I don't blame you for wanting to protect this little girl as much as you want to protect your little boy.
No one should be questioning or belitting you for your concerns. It is better to be concern than to brush it off. That is how children go un-detected when they are abused, raped or exposed to things way too soon.
You are not telling them how to parent. You are expressing concern for what she has said to your son.
I do suggest you discuss this with her parents. At least letting them know she asked for sex and a simple question of "Does she know what sex is, so my son can tell her no with out causing any confusion as he is begining to know what it is as well etc?".
If these parents are immature and get upset, I think it is a clear sign to keep your son away from play dates. As far as inappropriate watching (5 year old watching her 2 year old sibling), that would be in the hands of CPS at that point. If you see something or hear something (not rumor), you can report it.
Sometimes you are only limited to your child as there is only so much you can do. Only prayer is your next step she will be okay.