"Les garçons et les filles mangent."
Translation:The boys and the girls are eating.
The word 'les' indicates the plural form. When you have the plural form of 'fille', it becomes 'filles' ,with an 's'. In English you would say "one girl", but "three girls", 'girls' with an 's'. You wouldn't say "three girl", it would be grammatically incorrect. It's the same idea. Anytime, you have 'les', it becomes a plural form, and when it's a plural form, most of the time you add an 's'. There are of course, exceptions, as there are with any language, but for the most part, you just add 's'.