Learn Italian: Questi bambini sono molto giovani. - These children are very young.

Questi means "these" and is the masculine plural form of questo (this). We use it here because it has to match the masculine plural noun bambini (children). If we were talking about a single child, we would say questo bambino.

Does bambini mean "boys" or "children"?

It can mean both! Bambini is the masculine plural form of bambino (boy/child). In Italian, if you have a mixed group of boys and girls, or just children in general, you always use the masculine plural bambini. If you wanted to say "these girls", you would use the feminine plural: queste bambine.

Why doesn't molto change to molti to match the plural bambini?

When molto is used right before an adjective (like giovani) to mean "very", it acts as an adverb. Adverbs never change their endings. If molto were used to mean "many" along with a noun (like "many children" — molti bambini), then it would change to match the noun.

Why does the adjective giovani end in -i?