Well he was identified and there were pics and videos of him in custody. They wouldn’t likely have allowed that unless they were thinking of trying them as adults.
It was probably illegal to do that even if they were “thinking” of treating them as adults. The cops don’t decide that on their own. The issue of course is whether they have the maturity to be treated as adults. Good argument for the 17 year old, assuming he’s a normal 17 year old. Very unlikely for the 12 year old.