"These are children."
Translation:To są dzieci.
[This/That/It] is, [These/Those] are - all of those are dummy pronouns that translate as "To" regardless of the grammatical gender of the noun they refer to. And a "This is Y" construction takes Nominative.
"Te są dziećmi" is technically a correct sentence, but it means "These ones are children". Not the most common thing to say.
If the sentence begins with "To" (in English: This/That/These/Those - doesn't matter in this situation), which is then taking the role of the subject, and not a pronoun (therefore it stays the same regardless of the gender, both in singular and plural), the only option is "To jest/są Y" with Y in Nominative.
If at the beginning we had "They": "They are children", there would be two options:
Oni to dzieci, with Nominative
and Oni są dziećmi, with Instrumental.