you make a point by expressing your appreciation of THEM.
You may harbor resentment since the father dumped his responsibilities in your lap. It shows up in the ways you interact with your kids, such as inpatience and irritability.
Gratitude is a virtue. Many people do not like to express it because they think that would burden them with reciprocity. If you keep reminding your kids how much you are doing for them in order for them to feel indebted, they'd be reluctant to go along.
Leave yourself out to start with. Ask them to appreciate what they already have. Remind them of how lucky they are to be born in this time and place. Ask them to say "Thank you" more often. Learn how to write a "Thank You" note (there are books on this. Admittedly, these are father's job.)
When they bring up how caring their friend's mother is, say "Yeah, I'm sure there is a lot of love in that family. But we have love, too. I love you very much, that is why I am working so hard to hold this family together. I need you to do your part. Without the family, without each of us, there is nothing."
Treat yourself better; do not try to engender sympathy from them. Others treat you the way you treat yourself. Pamper yourself once in awhile, and show them how to do it.