I think some people are confusing the ability to tell right from wrong, with the aptitude to understand the gravity of an action - which is why children are not deemed fully culpable.
Most children aged 10 will know that a pan can be hot, or playing near roads is dangerous, but they are particularly dangerous environments for children because they make immature / irrational choices.
We as adults sometimes make similar bad decisions, but by the age of adulthood, we are deemed to have sufficient capability to rationalise quite well (on the whole). Children do not yet have that ability. They can show remarkable levels of maturity sometimes, but are far more prone to persuasion, fear, lack of perception etc.
The issue of premeditation is a much more complex one in a child, as 'setting out to take a child' or 'to harm a child' isn't quite the same (in a child's mind) as the 'murder' we as adults would interpret it as.
Imagine a child who wants to play with fire. To the child, it's a curiousity about fire, not a desire to commit arson per se. We know that lighting a match and seeing what it might do when it burns something is so dangerous, but the child doesn't always comprehend that. When the house burns down, the child genuinely did not set out to burn it down, nor did he comprehend the consequences of setting fire to the curtains. Events 'escalate' quickly.
We might never truly know what went through their minds on that day (they might never know themselves), but it is not unimaginable to think that events got ahead of them. They had been shoplifting earlier in the day, playing truant from school and stealing batteries and paint. They attempted to 'take' another child a little earlier, but failed, then came across an opportunity with James. It has been suggested they had talked about pushing a kid in front of a vehicle on road. Horrific in the eyes of an adult, and at least nasty in those of a 10 year old.
Did they intend to kill? we might never know, but if they were so keen, they equally had ample opportunity earlier to kill - yet they didn't.
Without going into too much detail, there were incidents along their journey that ebbed an flowed between bullying / cruelty and not knowing what to actually do with a child. In my own view, this is where things escalated in the irrational minds of two children.
At least one of the children had witnessed both parents attempting suicide, and violence in the home amongst siblings and parents - including knife attacks and alcoholism and the total loss of a home due to a fire. The other had two siblings with learning difficulties, and already had learning difficulties of his own, along with a reputation for aggression in the classroom. The three young siblings had been left alone at home a few years earlier which required police intervention to ensure their safety.
This type of background for young children doesn't necessarily equate to a sure fire pair of killers, but it does expose a troubled history and suggests in part, they were not your average pair of loveable young rogues. These were children bordering on feral. They weren't born that way, they became that way.
What they did was horrific. Why they did it is confusing and contradictory. But I think we at least have a small glimpse of how they came to be that way.