「彼には 子供が 六人 います。」 - "He has six children."
Have you REALLY compared the two sentences? While "He has six children." is a valid translation of the Japanese sentence, it is nowhere near a literal translation.
"He has six children." The subject is 'He'.
「彼には子供が六人います。」 The subject is 「子供」. Therefore, it is directly followed by the subject marker 「が」.
Last time I mentioned this fact in another thread, some people here seemed to think I was crazy, but I will say it again with confidence. "He has six children." is just a "translation" and it is even a good one in the sense that it makes perfect sense in English, the target language of this translation. It is, however, not what the original says as far as sentence structure goes.
This 「に」 is actually fairly close in meaning to "at" the location marker. The original sentence literally means "As for at/around him, six children exist." Wouldn't that make good sense in and of itself? I am not asking if it sounds natural in English. We can always fix a translation to make it sound natural. All I am trying to do is to prove that 「に」 in the original is essentially the same as the English "at" in this context.
Whether or not the word "at" could naturally appear in the English translation that everyone would agree on is of a secondary nature.