Social media can be a good source, too, since many pre-teens use it (even if they are younger than the minimum age to sign up). I've seen video diaries and interest related personal videos made by kids, for their friends and peer groups on YouTube. Also, reading the comments on videos that are largely of interest to that age group can be helpful, depending on what kind of stuff you're looking for. Blogs are another way of observing kids interactions (if they have any) and gaining an insight into their lives. Even blogs written by moms of kids in that range can be helpful. (I feel like I should add, although I think we all know this already, that nothing tricky or underhanded should be done to access these things. If it isn't available for the general public to view, you should just find another source that is.)
And reality tv shows or news programs that feature kids are another potential source, especially if you can find some related to your story idea (like, for insight on a child dancer you might watch Dance Moms and focus on the girls and what they say (as well as other programs that document young dancers, perhaps Dateline or 60 Minutes or 48 Hours has done something, etc.); you could also then look for interviews or even extra videos online since many shows don't put all their footage into an episode and will sometimes share extra bits on their website. Just listening to the kids talk can help, really, whether they are talking about something related to your idea or not.