bathroom “humor”/sillyness

My 9 year old daughter (Dec. birthday) can be inappropriate at times with her younger brothers (3, 4, & 7). For example, chanting nonsensical (imagine out of control silly behavior). She also toes the line with what is appropriate with the body. For example, this afternoon while they were outside she showed her brothers her bare bottom vice versa in a silly way. As a family we have standard that the private parts of the body (what is covered in a swimsuit) are private. We also speak of the body as something to be honored and taken care of. It bothers me that she is doing this at her age. She does not act like this in public settings or with friends. However, her younger brothers look up to her and I doubt they will make the same distinction. I want to trust that I can have them play in the backyard appropriately. I feel like I cannot. There is no history of abuse, she is being silly. I need advice on how to deal with it without making it a huge deal that brings more attention and curiosity/secrecy. We told the children that we need to see appropriate behavior at home and if not, playdates will need to be postponed. We limit tv and media including books, but it does bother me that they see "bathroom humor" in books and movies ocasionally and hear it from peers. I did cancel a playdate for tomorrow, each child did a correction and we dropped the subject.
I feel like my daughter gravitates to what is "off limits" and that bothers me. For a small example she will say things like: "oh I LOVE coffee!" since it is an adult drink and she knows she is not to have any. I feel it's a type of negative attention seeking.
Thank you Nicholeen! I appreciate your guidance!!