"We have children."
Translation:Nous avons des enfants.
in English, "we have children" is equivalent to "we have some children". The word "some" is implied. In French though, you can't imply that; you have to explicitly say it. "Nous avons des enfants" ("we have some children" is grammatically correct; "nous avons enfants" ("we have children") isn't. It's just one of those things.
"les" = the. so les enfants = the children where as "des" = some/any. so des enfants = some children ( also just an extra thought: both les and des are plural because the word children is plural if a word is singular, you would use le or la for the, and du or de la for some/any.)