Of course, they don't actually arrive to help until the final volume where they arrive just in time to save Kakyuu, Sailor Moon, and Chibi Chibi from the Soul Gardener, Sailor Heavy Metal Papillion. Chibimoon uses her final attack, Pink Lady's Freezing Kiss, and the Amazons use Amazon Jungle Arrow. Papillion is done away with and they all proceed to have a conversation about who on earth Chibi Chibi is. Chibiusa denies that Chibi Chibi is either her little sister or her daughter. The Amazons agree that each generation only gives birth to one princess. Chibiusa says that she just knows that Chibi Chibi is not her daughter from the future.
They don't have much time to discuss this, because Chi and Phi show up and take out Kakyuu in short order. Sailor Moon destroys them, but then the senshi (under Galaxia's control) show up and confirm that they are the next enemy that must be passed. Oooh. Sucks a lot.
The Amazon Quartet are wasted by the senshi's Galactica attacks trying to protect Sailor Moon and they fall to the ground unconscious. Usagi doesn't want to fight her own soldiers. She doesn't think she can. She loves them too much. Chibimoon wants to intercede and help fight. If Sailor Moon won't fight, they'll never get to Galaxia and won't be able to set things right. Chibi Chibi stays her hand and Sailor Moon regains the will to fight on her own and cleanses the senshi with Silver Moon Crystal Power Kiss. They turn to ash in front of her and their sailor crystals disappear.
Galaxia appears and lords over Usagi for a while, then Usagi plows into her, fed up with the crap she's taken from Galaxia. Chibi Chibi and Chibimoon follow her into the palace at Zero Star. Usagi and Galaxia have an exchange, then Usagi watches helplessly as Galaxia throws all the Sailor Crystals she's amassed into the Galaxy Cauldron where they return to elemental particles and so ends the immortal lives of her reincarnating Sailor Senshi. Chibimoon and Chibi Chibi watch horrified from a short ways back.
As they watch, Galaxia throws Mamoru into the cauldron as well, and Chibimoon fades from existence.
And that's the end.
Not really, but it almost is for our Sailor Warrior of Pink.
Sailor Moon finds the power and love inside of her to make everything right (not surprising) and she defeats Chaos and Galaxia and enables the rebirth of everyone. All the girls and Mamo-chan appear around her in the Galaxy Cauldron and Chibiusa is among them. She touches Usagi's face and then glides away on angel wings after saying she'll see Usagi in the Thirtieth Century.
One of the final thoughts of the manga occurs when Usagi and Mamoru are getting married. Usagi exclaims that she feels a bright light inside of her and that their daughter, the pink sailor soldier, will soon be born.
And with that, we have come full circle with Princess Usagi Small Lady Serenity, often called Chibiusa, Heir to the Throne of Crystal Tokyo, one time Pretty Suited Sailor Soldier in Training, protected by the future moon, and Sailormoon's one true Beautiful Dreamer.