I don't have kids (yet), but I would start with this, expressing it for a child, with words that s/he can understand, but not like a little baby, but a human.

Sex is an expression of ourselves. Sex is connection with yourself or with other/s. Sex is not bad, nor weird, nor a taboo.

We are different (boys and girls), but we feel the same when we touch, look, taste, etc. Explore yourself throught sex. Feel your body, and know it well. Deep, any and each part of it. Then, later, you can be ready to explore and sense another body, and you'll understand there's no difference.

Pleasure is only one of the feelings you van sense.

This is only a sumup, but from here, expand. Never make it a problem, or a taboo. Sex is not bad. Is our society and culture who made it look bad.