they (groups of people with at least one male)
they (groups of non-people, children, or all females or other non-male genders)
When can pronouns be omitted?
1st and 2nd person can always be omitted unless there is a reason to use them for emphasis
3rd person can be omitted if context is unambiguous. 3rd person shows the gender so if there are multiple genders or the subject is unclear the pronoun is necessary.