Ok. If you are here, then you frankly don't care about spoilers. And you want to know who Sailor Chibi-Chibi really is. Well...here goes.
If you've seen Sailor Moon Sailor Stars, you've heard of star seeds. Just what are they? Apparently, they are a person's crystalized lifeforce. Everybody has one. Scouts have especially powerful ones.
Sailor Galaxia used to be good. She was (and still is) the strongest sailor scout in the galaxy. Her most well-known accomplishment was sealing away Chaos. Unfortunately, as the saying goes, "absolute power corrupts absolutely." This brave deed backfired on Galaxia to some degree, since she started to be overtaken by Chaos. She realized this, and, as a final effort before Chaos had completely taken her over, she put her hope and goodness into her star seed. She then sent this good and pure star seed into space, to find someone who represented love and justice to protect it.
The star seed landed on Earth, found that person (Sailor Moon), and thus became a toddler version of its protector...hence, Chibi-Chibi.
As the Animaniacs say, "And that's all the facts."