I think this comes down to a matter of upbringing or locale. "Imiko" is a miscarriage or stillborn, which some people do call a "dream child" as in 'child you only meet in your dreams'
Wow I randomly read that comment but wow.
Maybe even if they do call it that, in English "dream" has two different meanings, so you're saying that instead of extremely positive meaning as in the child so good/great, nice or talented, it's the tragic meaning, huh?
That means we have there something even worse than not explaining the word. That's just misinformation. TL note corelate it with the English "dream child" with positive expectation, but it's the opposite.
What's more is simple googling gives you explanation
Definition of 忌み子: いみこ - imiko - unwanted child, shunned child.
Written with characters 忌 meaning
mourning, abhor, detestable, death anniversary.
And 子 meaning
child, sign of the rat, 11PM-1AM, first sign of Chinese zodiac
At least
according to this site.
So even without the TL note, curious people would get a better idea what it means without being misinformed, misled and confused of what it means.