Here's why:
* Gerund: A gerund is a verb ending in "-ing" that acts as a noun. In this sentence, "singing" is the gerund.
* Gerund Phrase: A gerund phrase is a gerund plus any modifiers or objects that go with it. In this case, "children" modifies the gerund "singing".
The entire phrase "children singing" functions as the object of the preposition "of" in the sentence.