Final answer:
The pronoun to use when there is one boy and the rest girls is 'Ellos' in Spanish.
Step-by-step explanation:
The correct pronoun to use to say 'they' when there is one boy and the rest girls is Ellos. In Spanish, the masculine pronoun 'Ellos' is used when referring to a group of people that includes both males and females. So even if there is only one boy in the group, we use 'Ellos'.
Learn more about Pronouns in Spanish