It is more likely that these kids are getting their notions from overhearing parents or friends of parents or siblings talk about relationships and playing "grown up". Rest assured that it is pretty unlikely a pre-k girl is ACTUALLY thinking about dating and what it leads to the way a girl would in high school or middle school.
My first "boyfreind" was when I was five! Everyone who just went "oh my!"... I say, "whatever!" We threw rocks at each other and tried to escape the playyard at our preschool, and we held hands just like "big kids". It lasted a grand total of an afternoon, and was a game- like everything is to kids that age.
Kids emulate the people that surround them, especially adults. My 2 year old LOVES make up and will "play make up" and be a "pretty girl" all day long. She also likes to pretend she is a dog or a cat, but it doesn't make me think she is dillusional.
Make sure the relationships these kids are emulating are healthy relationships, by paying attention to who your kids are listening to- and relax.
As far as shaking thier hips- that is what highschool cheerleaders do. These girls aren't trying to be "sexy"- and if someone thinks a little girl IS sexy when they shake thier hips- I think that person is the one with the problem!
Adults thinking kids are too "sexy" are at least half the problem. The kids are being kids. Back in the twenties girls who were just toddlers wore dresses so short that you could see their ruffly panties! (how scandolous!)- these kids were not being sexualized- no, they were being cute and appropriate for their age!
Adults who MAKE it into an issue by reading things into normal little kid's behavior- are making a problem exist that shouldn't.
I shook my hips dancing to music (shania twain) as a little girl, and it was because it was fun. I still thought boys had "cooties" most of the time. If someone says these girls are acting too sexy- the answer is simple, a girl who is too little to even have a dash of puberty hormones- is not, and will never be "sexy"- even if they are running around buck-naked! The person who thinks they are- has the problem!
Irregardless- good question!
-M.
(by the way, you are a good parent for caring about it, of course!)