AR parents were told by AR he was off to try and kill. Twice.
These were unrelated incidents though, right? It's not been suggested that they had any knowledge of what he'd planned to do that day.
They paid for and received the weaponry, because he was too young himself. No police called.
Again, this was a moral failing. It's agreed that they knew he was generally capable of harm, but not of any specific plans he'd made for that day.
I suspect they won’t be charged because no one can decide what with
It's not that no one can decide what to charge them with, it's that's there's nothing to charge them with.
It's not illegal to not report a crime, or suspicion of one, in almost all cases. An act of terrorism is one of the very few exceptions where not reporting is an offense in itself. The Southport attacks weren't classified as terror-related, but even if they were, there's no evidence that the parents had any knowledge that he was planning or conducting these crimes.