As others have said don't worry about kids not loving you as much. You will always be their father and they will love you.
I'm not sure if you mentioned before but why do you have to see them at her place? Can you take them out (maybe to dinner or to a park)? I can't imagine how hard it is to enjoy the time with the coldness it sounds like she shows you.
Have you guys discussed custody arrangements? Sorry again if I missed this.
Personality is who the world sees, character is who you are
