Does it come only from the Legacy tv series cluster ? They are almost entirely wrong in that tv series. That's the only portrayal I know of that represents Kitana as a baby when Kahn invaded.
In Sindel's original ending for MK3, Kitana is shown as a little girl (10-12 years old ) during Kahn's invasion of Edenia.
More evidence that she was not an infant comes from Rain's ending in MK Trilogy, which implies a connection between him and Kitana (unknown if friendly, romantic or something else) before the Edenian War.
Kitana was portrayed as a teenager during Shao Kahn's assault on Edenia in the non-canon Defenders of the Realm series, and as much as I'd hate that animated series, that's the best representation of Kitana in the Edenian days
and similarly in the MK Conquest series, where she reveals that her father had trusted her with Edenia's essence before Shao Kahn took the Capital.
In the canon games, Kitana recognizes her mother Sindel both in Mortal Kombat 3 and in the Mortal Kombat reboot from 2011. She has memories about her mother. She could not have had these if her mother had killed herself when she was an infant.
She also recognizes her mother in the non-canon movie.
The possible answer for her submission to Shao Kahn was given in the non-canon adventure game “Shaolin Monks”, where she was placed under Kahn's sorcery mind spell to obey him and not remember her past. That explanation was not well received by the fans back then, but it certainly seems like a good one after some years have passed. It could also explain why in the 2011 reboot she has absolutely no recollection of growing up with another Mileena clone, though Jade seems to know such clones exist and attempts to stop Kitana from venturing into Shang Tsung's Flesh Pits where a new Mileena is born. This explanation even works well with the non-canon ending of Mortal Kombat Conquest where Kahn punished her for her failed rebellion.
Whichever explanation you accept (that she knew all along who her parents were or that she was under a spell to forget her origins) she certainly wasn't an infant when Edenia was attacked. And considering that the Edenian Resistance and Protector God Argus held Shao Kahn for quite some years after the loss ot the 10th tournament (enough for Rain to grow from an orphan boy to a young warrior), she may have grown up into a beautiful young woman during the War.
But why is this convention, that she was an infant? I've seen this stated many times (but only in the last few years). Is it all because of that awful Legacy episode with the unmasked Kahn and the orcs from TLOTR?
Sorry if I posted this multiple times.