"The child likes to eat fish."
Translation:Đứa trẻ thích ăn cá.
A plural marker isn't necessary because you are talkinf about the noun in general, so I believe your surmise is correct.
Also, you can leave out plural markers for your previous question. Vietnamese is a somewhat lazy language in that you can leave out words like "the" and "a(n)" and still make a coherent sentence.
Hope that helps!