That is incredibly sick, and I would say it is in no way normal. It does however lead me to ask what sort of upbringing have those boys had to even know what sex is?
Not knowing the laws of the area it is difficult to say whether they can legaly held responsible, but in general I would answer no, as the person has to understand that what they did was wrong, and how it was wrong. And in general an 8 yo is very unlikely to understand what they did, let alone that it was wrong.
In answer to the last question legaly the person who did the act is responsible for it, but in this case I would be more inclined to blame the parents. Especialy since they tried bringing up the defence that it was consentual, because as already stated, an 11 yo is very unlikely to consent to sex.