As AndrewGrimm notes, "children" has two very distinct meanings: It can refer to people who are not yet adults, or it can refer to people who are the offspring of a specified person or people.
It is normal and common to refer to adults as "children" when expressing the relationship. Saying that so-and-so are the "children of" someone is another way of saying the "sons and/or daughters of". For example, "Alice and Bob are the children of Carl." "Children of current employees are given preference in hiring." Etc. Depending on the context, you may or may not specify that they are adults. You would not say "Alice and Bob are children" without giving an "of" someone if they are adults.
Side note: "Children" (without a relationship) can mean anyone who is not an adult, but it is unusual to use it to refer to people who are more than 12 or so unless you are including them in a group that includes such younger people. That is, it would be odd to refer to a group of 17-year-olds as "children". It is more common to call them "teenagers" or "young people". But if you were talking about a group of people ranging in age from 6 to 18, you might refer to them collectively as "children". Like, you might say "the children in our school district". But few would say "the children in our high school".