You can use 'their' when you are talking about both males and females.
Each girl must make up her own mind.
Each person must make up their own mind.
Each person must make up his or her own mind. (It sounds a little formal.)
Plural pronouns can go with everyone, everybody, someone, no one, etc.
Has everyone finished their drinks?
Somebody has left their umbrella behind!
No one here seems to know what they are doing.