how do i keep girl body fit?

Answer 1

you must pay candie coins to get a trainer for a limited time, else, you must do it manualy

Answer 2

Segadors answer only applies if you are happy with the girls current stats. The personal trainer will keep the girls at the stats they are, but will not improve them.

First you train the girls manually until you are satisfied with their stats. Then you can hire the personal trainer to keep the girls in shape.