Fortunately Voxer uses the TLS/SSL for encryption which is the strongest there is so hopefully it is not easily possible to hack someones Voxer account.
There have been cases of bullying on there, as with most mobile apps, so precautions should be taken and parents should be aware that this could be a problem. However, with cyber bullying cases making headline news, more kids are being charged and arrested for their actions online which can only be a good thing.
By default the Voxer app enables 'share security' and disables the 'privacy mode' so it is best to change this in settings - location OFF, Privacy ON.
It is also worth pointing out that hacking is illegal and, in some cases, does lead to imprisonment. Everyone should be able to keep their information private and authorities are cracking down on hackers luckily.