I will advice that you focus more on your relationship with your children. Be there for them when they need you. Give them the best of your love and attention when they are with you. You will influence them profoundly if you make the best use of the time you have with them. Though they have fun when they are with their dad, it does not give the impression that they don't enjoy your company too.
So, instead of getting uptight whenever they are away, take time to develop yourself; read good books that will add value to you, get some training, and attend seminars. Reflect and look for ways of further improving your relationships. One thing you must realize is this, when they are with you, it is usually serious business, school, chores, etc. But when they are with him it is holiday time and what do you expect, just leisure. So you have to introduce some measure of leisure into your routine with the children too.

It is wise that you are concerned about your daughter's attitude towards boys. The truth is, it is the father-daughter relationship that lays the foundation for a girl's future involvement with the male gender. Every girl looks up to her father as the “ideal man” and she wants to have a boyfriend who is like her father - strong, caring and loving. She sees herself through his eyes and will believe anything he says. The truth is, daughters and women in general derive their identities from their fathers. They look to their fathers for definition and affirmation.
I advice that you make time to befriend her enough to build her self esteem, so that no man will be able to manipulate her in future. Then she is secured in herself and her involvement with men will not be approval seeking. Also this will help set her priorities in life and the issue of academics will automatically fall in place.