During issue 80 of Supergirl, Supergirl (Linda Danvers) went back in time to Earth-One (pre-Crisis on Infinite Earths) where she met and fell in love with that era’s Superman. During that time she bore Superman a daughter, Ariella.
When The Spectre came to take her back to her original timeline, Linda made the ultimate sacrifice leaving her daughter and the man she loved so that Kara could come back to her time, even knowing that she would die during the Crisis on Infinite Earths.
Linda bargained with the Spectre agreeing only to go back to her original time if her daughter, Ariella, would be spared. Agreeing, the Spectre wiped all traces of Linda Danvers from history, altering Superman’s memories of her so that he never remembered meeting her, let alone having a child with her.
Ariella has been seen several times in current continuity, she appeared in the DC One Million storyline as that era’s Supergirl where she was seen destroying an alien armada, albeit by accident, and had apparently "adopted" a new father figure.
Most significantly though, she was last seen with Klarion The Witch Boy, who seemed to have developed a crush on Ariella, though she was still only 6 years old.
Though Ariella is only 6 years old, it is wise not to judge her by age or size. She has shown signs of powers that surpass Superman and all previous "Supergirl"s. Though she has not spent a large amount of time in the current continuity, it is assumed that she is still searching for her mother and father (if she remembers them at all). When Linda Danvers left earth it was assumed that she went searching for her daughter though this has yet to be said in the comics.
Ariella’s whereabouts are currently unknown; her ability to travel through time at will and immunity to changes in the timeline make her a very dangerous adversary. We will more than likely see her again sometime; hopefully, she will find her mother but until then she seems quite happy to explore space with her new friend Klarion The Witch Boy.