Learn Spanish (Spain): A los niños les gustan los deportes.

With verbs like gustar, the thing being liked is actually the grammatical subject. The sentence literally means "Sports are pleasing to the boys." Since los deportes (sports) is plural, the verb must be the plural form gustan.

Why do we need both A los niños and les? Aren't they repeating the same thing?

Yes, they are! In Spanish, you must use the pronoun les (to them) with gustar. Adding A los niños (To the boys) at the beginning is just to clarify exactly who les refers to, since it could also mean "to you all".

Why is it los deportes instead of just deportes? We don't say "the sports" in English.

When talking about things in a general sense, like liking sports as a whole, Spanish requires the definite article (el, la, los, or las). So, you always say you like los deportes rather than just deportes.

Could los niños also be translated as "the children"?

Exactly. While it is translated as "the boys" here, los niños is also the standard way to say "the children" when talking about a mixed group of boys and girls.