I was honestly shocked to read this. Can it be surprising the kids think it's okay to steal if this is their parent's response? Clearly two very smart guys but in the long run ethics matter a lot more than money or intelligence.
In other words: parents raising kids giving them values like respecting the private property of others (no matter if it's actual physical gold coins, a bicycle, Bitcoins or whatever really) probably wouldn't use such pathetic tactics to try to dismiss the case. But the real point is: parents raising kids with actual values probably wouldn't be raising scummy thieves in the first place.
However, aggressively defending one's legal rights is also morally justified, esp. in Common Law jurisdictions with adversarial procedure. In fact, we'd likely have a more just (and thus more moral) system if more people asserted their legal rights in criminal trials, and took matters before juries rather than prisoner's dilemma plea agreements. (I understand why they don't, which is why plea agreements are so pernicious.)
More likely than not, the bitcoin is gone and the parents don't want to be on the hook for a substantial sum of money. Meanwhile, their kid is grounded for ever.
I wonder if HMRC would be interested in what would have been a considerable capital gain which likely went unreported for tax purposes.