Don't try to make up for what their father does. Go ahead and make your own plans and follow through with them.
You have no control over what your jerk of a stbx does. He is responsible for his relationship with his kids and keeping the promises that he makes. He is also responsible if he breaks those promises.
And one day, he will realize just what he threw away. Because one day, the kids will be able to choose not to see him. And there isn't a court in this country that will order an adult to visit their parent.