It is impossible to know how we would feel if we were their parents. We can only try to imagine how we would feel if our children did such a thing. Most of us can probably say that are children are unlikely to because they feel loved, are happy, safe, secure and children who experience these things do not commit such horrific acts of violence against other children.
How do we know their parents cared at all, they might have been glad to get rid of them. They might believe, as some, do that they were born evil and it had nothing to do with them as their parents. This might have been because they had had little love themselves and were unable to give their children what they needed.
Thats why in order to prevent things like this happening we have to look at why those children did it, why they wanted to, what happened in their lives. Its especially important in the first 3 years, when they need to be nurtured in a warm, caring home in order for their brain to develop properly, in order to develop empathy. Then we need to help, support and educate the parents who cannot do this before its too late.
The answer is not to have the death penalty or longer sentences, that does not make a damaged child, full of rage change their mind about doing it.