Note: this answer contains spoilers for both Blade Runner and Blade Runner 2049.
I don't know if you are aware of the multiple cuts that were released of the original movie. The theatrical cut ends with the couple (Deckard+Rachael) running away, but it lacks some details that Ridley Scott had planned. Later on, he released the "Director's Cut" that changed everything.
If you consider only the theatrical cut, then the child of both is a hybrid, but the director's cut reveals that Deckard is, in fact, a replicant. There is a scene where he dreams about a unicorn. The last scene of the movie shows him noticing an origami unicorn on the floor, left there by another cop. Dreams are one of the keys to determine whether someone is a replicant. As they are implanted, real people know the dreams the replicants have. I think there are some issues with the original plot to determine that Deckard is a replicant, but at the end of the day, I can't go against Ridley Scott himself, who confirmed this theory.
Moving to Blade Runner 2049, now you know the child is born from two replicants. So Deckard, just as Rachel, is a special creation of Tyrell (he even gets old!). Denis Villeneuve avoids addressing this with certainty, so that Wallace could try to recreate Rachael and fail. But he hints that Wallace suspects about Deckard being a replicant when he captures Harrison Ford's character.
So Rachael is not the key, at least not alone. Both are the key, or they form a combination of key and lock.
The child, then, is the combination of both, the fruit of life directly reproduced by replicants.