"Your future child will be standing there."
"The person standing over there is your future child."
I have no idea what Wanikani is, but its TL is much closer to the original than yours is and here is why.
The subject of the original sentence is 「あそこに立っているの」. The 「の」 nominalizes the verb phrase that precedes it. Thus. the 「の」 means "the one" or "the person", and that is the subject of the original sentence.
The subject of Wanikani's TL is "the person standing there", which is exactly the same as in the original sentence.
The subject of your TL is "your future child", isn't it? That is already very different from the original.
Let's take a look at the predicate in the original, which is 「あなたのみらいの子どもです」.
That is the predicate in Wanikani's TL, too -- "is your future child".
The predicate in yours is "will be standing there", which is different from the predicate in the original.
Hope you are following all this. (Since you are new, I have no idea about how much Japanese you already know.) You have got the sentence structure wrong this time.
what would be the correct translation of "Your future child will be standing there" in Japanese?
That would be something like: