(It's difficult with adults who disagree with us, too. of course, because while they object to our "irrational" absolutes, they refuse to see that they too come from a place of unquestioned and often ill-thought out principles, and I've said repeatedly that one of the silver linings in the glowering thunderclouds of our current political climate is the clear evidence that when push comes to shove very few parties or persons hold to their stated principles. Oh, they have principles -Freedom! Dignity of human life! Tolerance! Godliness! self-determination! Safety! - but the ones they use as talking points are not the ones they actually hold, when it comes to applying them to other matters than their pet causes. But that's another topic...)
With children the notion of positive and negative are often completely linear, and they struggle to comprehend how evil exists if a good God created everything, and they reject assurances that evil is not equivalent in power, that God is "All", that the demonic is not something they need to worry about if they hold fast to Him.
(And thanks in part to their age, and in part to the super-abundance of horror films, games, books and graphic novels, and mostly thanks to their society and families having left a vacuum where thought of God should be, they are fascinated by the devil. But that too is another topic.)
They need a different geometry through which to ponder God and Virtue and Goodness, Truth and Beauty, they need to see existence more multi-dimensionally, they need to see that their is more than one direction away from the center, from home, from God; so that the nearer we draw to virtues, actual virtues themselves rather than the actions to which those virtues might prompt us, the nearer we find ourselves to God.
Because all else is Nothingness.
And they, WE need to discern the difference between abundance and excess.
“The modern world...is full of wild and wasted virtues...it is not merely the vices that are let loose...the virtues are let loose also; and the virtues wander more wildly, and the virtues do more terrible damage. The modern world is full of the old Christian virtues gone mad. The virtues have gone mad because they have been isolated from each other and are wandering alone. Thus some scientists care for truth; and their truth is pitiless. Thus some humanitarians only care for pity; and their pity (I am sorry to say) is often untruthful.”
-- GK Chesterton