What is the meaning of 忚 and how is it pronounced? -- (a logical argument)
忚 (u+5fda) is rarely used in the past, it did appear in several books.

the latest one, with english explanation, was in early 18th century, in robert morrison's "a dictionary of the chinese language", volume ii, part i (page 139 of pdf file):

㦒(u+3992)忚: insultingly neglectful

https://archive.org/details/p1dictionaryofch02morruoft



then, its' earliest appearance in literatures of yore, was in 輶軒使者絕代語釋別國方言　卷十, by 揚雄, of 漢 dynasty (page 79 of pdf file):

眠娗﹒脉蜴﹒賜施﹒茭媞﹒譠謾﹒㦒忚﹒皆欺謾之語也﹒楚郢以南﹒東揚之郊通語也

"欺謾" is roughly deceive, disrespectful

https://archive.org/details/06050580.cn



its' next appearance was in 廣韻　上平聲　卷第一﹒齊韻第十二 (page 31):

忚　欺謾之皃 (貌)

https://www.digital.archives.go.jp/DAS/pickup/view/category/categoryArchives/0500000000/0511000000/00



back to your question, i don't think that this character was used in 論語.

康熙字典 was published in 1716; so it's about 300 years old :)

about the argument:

Now, therefore, the only (or most) logical meaning
of 忚 is [he/she/it] for children and babies.


well, it's not. i would say that 

他　he 也﹒從人﹒也聲
她　she 也﹒從女﹒也聲　
祂　god 也﹒從示﹒也聲　
牠　it 也﹒從牛﹒也聲　


so, these four are 形聲字.

lastly, i think that most 小學類　字書 had the character "忚", and, explained it; though in literary chinese, and printed in traditional script.