A useful Scripture passage might be Mark 10:18: “Why do you call me good? No one is good but God alone.”
Of course by “good” he means “pure good,” i.e., “goodness itself.”
In terms of explaining it to young folks… well, have you already covered God’s omniscience with them? That God knows everything perfectly? You might then explain that God’s love arises from his knowing everything perfectly. A good example is making a new friend. When you meet someone for the first time you don’t necessarily know a lot about them and, consequently, don’t feel very strongly about them. As you get to know them more and more, your feelings for them intensify, whether the romantic feelings a man has toward a woman or the platonic, friendly feelings any two people can share for one another. Love in this sense is a form of knowledge. To love something is to know it and understand it and appreciate it. This is why, for instance, humans can rejoice in a painting but dogs can’t, because they lack reason and understanding.
So if love is a form of knowledge, then God, who is all-knowing, should be all-loving: that is, he should value everything according to its true worth.