I agree on the points already mentioned by Jeff and FB:
1.) the absolute necessity of stressing that God loves them and will never stop loving them. If you’re a trinitarian (and you should be ), be sure to stress that God gives Himself for all our sakes on the cross. (Not only on the cross, but the cross is emblematic of the extent God gives Himself for our sake.)
2.) the maximum-hyperbolic language of punishment should be downplayed for children; much like the violence and sex in the Bible. (Frankly, I prefer not to bring that up for non-believers anyway. The extreme language is typically directed in the NT toward those of us who are already believers. )
However I would also add:
3.) some notion of sin has to be given as well, insofar as you believe children can be sinners (and I certainly do.) Stress that just as God will always act in love even to sinners, He will also always be acting to lead them to love and be fair to one another. That may in fact involve punishment, as much or as little as God sees to be necessary. But God isn’t only dictating that punishment from on high. He shares the punishment with us, so that we can learn to trust Him–and simply because He loves us.
4.) I would also add that the reason we even can sin, is because God allows us to be real boys and girls and not just puppets. Whenever we do something wrong, we are always abusing the grace of God, as well as (usually) abusing other people like ourselves. Bad things happen because God loves sinners, too; even though He does not and cannot love the bad things we do. (The cross is strongly connected to this as well.)
While you don’t have to go into details, I recommend focusing on how these elements apply to you yourself in your own life: in your trust of God’s love; and in your acknowledgment and repentance for your sins, not only against God but against other people; and in your cooperation with God toward doing justice for those you’ve wronged, and in loving your enemies (even if they are the ones who have sinned against you).
And if you aren’t doing so well at the moment on one or more of those points…? Well, be honest about that, and let them know you see this as an opportunity for you yourself to begin doing better.
If some of them don’t believe in God?–I wouldn’t press the issue on that. Apologetics is part of evangelism, not vice versa. You’re giving a testimony about what you believe. If they can believe it along with you, then good. If not, then just ask them to believe as much of what you’re saying as they can believe, as much as they themselves see right to believe, however much that is. God accepts all of whatever little we can give in good faith.