The difference is between civil and criminal offences.
A civil offence very rarely, if ever, requires intent - what is known as "mens rea" (the guilty mind).
A civil offence example would be a speeding fine, or late payment of a government charge. There is no need for a court to prove intent.
With a criminal offence, mens rea is essential - and much of a case will hinge upon the "guilty mind" as well as whether the facts support the offence. The guilty mind is not a moral judgement, it is just a measure of whether there was intent to commit in consideration of the legislation in question. Judges are not there to prosecute based upon the moral "right", merely to execute laws in the light of guilt, or not. Politicians are the ones with the morals, apparently, in making the laws (eg it is wrong to starve a baby to death, we need a law to protect them, say the politicians).
Criminal offence cases are taken by the government (crown in the UK) and require the government to want/agree a case is due. Civil offences are taken by individuals against other individuals, in the main other than for minor things like traffic offences.
This leads to some oddities... a lack of "guilt" can lead to someone being proven innocent, and then prosecuted separately for a civil offence by an individual who was wronged, which typically involves payment of monetary damages and no need to prove guilty. This is pretty common in slander cases and the like. It is like double jeopardy.
It is the lack of a child to have an adult mind (in law, at least) that means they are unlikely to ever be prosecuted for a criminal offence - they are unable to form a "mens rea".
Hope this helps (going from memory here, it was a while ago I studied criminal law).