Babies are born selfish.
They want food and changed. They want to be waited on. They cry to express there dissatisfaction with the service they receive. They also coo and laugh and smile and otherwise show other emotions as long as they are getting what they want.
We feed and change them and they grow. As they grow they learn certain skills that enable them become more mobile and to communicate in more understandable ways.
We as parents have certain responsibilities toward our children. We are supposed to feed them, keep them safe, change their diapers, keep them clean, etc. These are things that society expects of us.
They expect similar things if we have a dog or cat. They get upset if we let our pets starve, get diseases, run wild, and they do the same if we neglect the upbringing of our children and don't feed them, leave them alone locked in a house or bathroom all day.
So, this is society's expectation. God expects us to also impart some moral and spiritual training also. We are to remove the selfishness. Most children also have a tendency to protect themselves from perceived danger. This danger being an upset parent. They don't like to be caught with there hand in the proverbial cookie jar. Therefore they tend to distort the truth or just plain lie.
Why? Because they know they have done something that they think or know displeases their keepers. They don't want the consequences.
This moral awareness is not full fledged awareness that an adult has.
But it is a transgression and they know it.
What happens next in the child's mind is based on the parents reaction and handling of this "transgression." This is where we teach.
The Bible is full of instances where we see the results of the parents strategies in dealing with their children. Because children are all different and because they may react differently to the same home situation we may have a "good" child and a "bad" child come from the same environment. Cain and Abel for example. Then, we as parents may treat children differently. Jacob and Esau and Joseph and his brothers come quickly to mind.
My thoughts are that as a child grows in physical stature, there is also a mental development and a moral development. That moral development begins even before a child can speak and is nurtured as a mother and father begin to say no about certain things.
The child grows. Knowledge and awareness grows. Responsibilities begin to be added to a child's life. It is gradual and everyday.
At first they are become accountable to their parents, then parents and other family members. Add an awareness of God that, hopefully continues to grow throughout their lifetime. Add teachers, guards, law enforcement, doctors, nurses, fire personnel.
Moral responsibility doesn't wait to age 5 or 12 or 16 or 21. It grows throughout a lifetime and though there may be principles we learn about it, the applications we face are many and varied and are presented to us for a whole lifetime.
Children don't have a moral awareness of an adult but they still have an early on sense of some things are right and some are a no-no.