It's very hard to make any generalisations from this case, I think, partly because we don't know very much about the upbringings and backgrounds of the two murderers.
When Blake Morrison (who wrote a book about the Bulger case) was interviewed on Today this morning, he said he thought that, while the internet/social media played a part in the decision to kill (girl X had watched lots of violent content), the bigger factor was that the two of them acted together. Neither of them would have committed murder on their own, but they egged each other on.
Similar cases include the Bulger case, the Parker-Hulme case (in New Zealand) and the 19th century case of brothers Robert and Nathaniel Coombes, who killed their mother, though only one was convicted.
It seems to me that although stabbings are, sadly, now common, cases such as these, which involve two people cold-bloodedly planning a murder, are exceptional. The peculiar and twisted psychology that leads two children to commit such a crime is hard to understand. I'm not at all convinced that better mental health policies would make any difference.