Even a twelve year old would normally not have the means to plan such an attack. Researching how to do it, getting the acid, actually going through with the plan – that sounds suspiciously like someone older planning it and having the children execute the attack because they'll face more lenient penalties from the law IMO.

The serious answer is they learn it from their parents and society at large.

I'd be willing to bet money that the household they grew up in has Fox News blasting 24/7. They certainly live in a conservative area, so all their friends are probably growing up in similar situations. When you feed children an informational diet of consistent hatred and violence, you can't really be surprised when they act on it.