Namine was born when Sora released Kairi's heart from within his own. Namine was made from Sora's Body/Soul and Kairi's heart, which is why Namine has power over Sora's heart, because she's was born of both him and Kairi
The Princess' of Heart, however, did no such thing. Their hearts were taken and forged into the Heart Unlocker keyblade, while their Bodies remained trapped in the normal realm. A Nobody can only be created if a Heartless is created, the Body and Soul are taken to the Twilight Realm where they are reborn as a Nobody
The Princess' of Heart are incapable of creating a Heartless because they hold no darkness, they are entirely pure, which is why even when their Hearts are taken they do not become Heartless, because a Heartless has to be born of Darkness.
Kairi did not produce a Heartless, as she is a Princess of Heart, the only reason Namine was made was because Namine was made from Sora's Body/Soul