"It" in the sentence above means (in Portuguese) "isso", "ele", "ela".
In English we usually need a subject to make up a correct sentence. The exception is, for example, when using Imperative, look:
Leave me alone.
Go to hell.
Open the door.
Even when we talk about the weather we have to use a subject:
It has just rained. "acabou de chover."
It snows a lot here in winter. "neva muito aqui no inverno."
The word "Children" in your question is plural. It doesn't fit in the sentence you mentioned.
Send more clues so we can explain it better, right!