Yes, there are some kids that do play with other kids of the opposite sex and yes, there are kids that play exclusively with kids of their own sex - this is not a black and white distinction, it's a gradable spectrum with numerous variables to it. For example, I would wager that children who have siblings of the opposite sex
(and good relations with them) are more keen on integrating with children of the opposite sex in other environments
(at least if peer pressure doesn't stand in the way), but I'd have to look for some research on that matter before stating it as fact.
I've got plenty of experience with children, I've had children psychology as a course in university
(I'm on a teacher's programme after all) and I've clocked several hours teaching English and Computer Science in my local primary as a part of work experience - I've met a lot of kids, I got to observe what they do on breaks and I know what I'm talking about.
Thing is, you cannot base your claims solely on personal experience - that's circumstancial and anecdotal evidence. Not only that, there's a myriad of development anomalies that can occur which may trigger an early onset of puberty, but those cases are rare and don't really reflect the general rule. You need to have a more all-encompasing look on the matter, read some related literature and delve into the scientific studies, this is a hightly active field in psychology, there's plenty of material to go by. I'm merely giving you the general rule of how this thing works.