Whoa! Kids do really ask hard questions, don’t they?
I would definitely agree with what @KMac has said here. I think we need to make an important distinction between human beings who might be enemies of God (and other human beings), and spiritual beings like Satan, i.e. the Devil, who just is God’s enemy. There are very different metaphysical categories going on here and I don’t think we should conflate the two.
Now, I’m not sure how to explain this to a child, but perhaps something like this might help:
Satan, or the Devil, is the evil angel who tries to make us hate God, and each other. He is the one who tries to make us to be enemies against each other, and God wants us to freely reject the Devil and his ways, and instead accept Jesus and His ways.
If we accept Jesus and reject the Devil, then we can learn how to love our enemies, and pray for those who hurt us. If the Devil had his way with us, we would never love our enemy, we would hate each other. Because the Devil rejects God, even though he knows very well who God is, he cannot ever love. So, if the Devil will never love God, then we can refuse to love him, because he just is everything that love is not. If we loved the Devil, we would be on board with his hate, which is neither good nor possible. We want to love people, so in order to love people, even bad people, we have to reject the Devil, because he is always trying to stop us from loving.
That may still be too technical for a child, but maybe it could help.