If you’re going to write children…
…please, please try and get their voice right.
There’s nothing that ruins a story faster than characters who talk (or subvocalise) in an entirely unconvincing way. Based on personal and anecdotal experience it seems sprogs, kids and teens are the demographic writers tend to struggle with the most. This is understandable, to an extent; youth culture has a pace of change more rapid than science fiction has been able to boast for years. But you don’t get to make excuses for stories.
I don’t have any answers for writers who want to try and write contemporary youth well, but I’d suggest that irreverent cartoons beloved of children and young teens, TV shows that actually feature young actors (The Imbetweeners and Skins spring to mind, though I’ve seen little of either), and paying attention to the way groups of young people talk and interact is a more likely route to success than some painfully forced artifice that doesn’t even closely resemble your own youth.
Overall, though, you have to remember that kids are just like any other people. They don’t habitually pepper their sentences with buzzwords and pop-culture references – though in certain contexts they might. Some kids might use embarrassing substitutions like “freaking” or archaisms like “naff”, whereas others will cuss with the best of them. Whatever. If you are writing a character you need to ground them in a context, and that context will inform how they think, act and talk.