1.The students are busy with their homework all the time.
=The students are busy (in) doing their homework all the time.
2.Millie is 12.
=Millie is 12 years old.
=Millie is at the age of 12.
3.They really have fun at the party.
=They really enjoy themselves at the party.
=They really have a good time at the party.
4.They are both good swimmers in the club.
=Both of them are good swimmers in the club.
=They are both good at swimming in the club.
=They both do well in swimming in the club.