The "Bad" Kids

I am not perfect.  My kid is not perfect.  We have good days, not so good days, great days and bad days.  I was not a kid that ran around the restaurant.  All I needed was "the Look" from Dad to shut me up.  I knew to please and thank you to everyone.  Could I be a brat sometimes?  Of course.
Even the best behaved children have their moments.  There are a couple of kids that Savannah sees and no matter what, during the time we are with them, every single time, without a doubt, these kids both have a fit, disobey their parents, fight, and get punished.  (and Vannah's age has nothing to do with it cuz one of the kids is her age).
Savannah is very well behaved when she is with them and I feel awkward sometimes when the other kids are getting disciplined and Savannah just stands there and watches.  And what is even worse is this is family, so there is no "not" hanging out with them.
My problem isn't so much the kids because their parents do watch them and discipline them, which is good.  But every single time, after they go home, Savannah turns into a little hellion!  She will talk back, gets an attitude, and misbehaves!  This only lasts a few hours up to a day, but she only acts like this after being around those other kids.  I also know its not all kids.  She is completely fine after hanging out with other cousins, children, or kids at the park. 
I'm not sure if she is seeing how they are acting and thinks it is okay too; but she waits till they are gone.  Its very frustrating and I now understand why my parents didn't like a few of my friends growing up; I must have acted like a little brat around certain kids.  Payback eh?