How often should my girlfriend be paying?

My girlfriend has two little kids. I am in transition from jobs finishing up school. I love her but this is starting to bother me. I was wondering if anybody has any advice here. I am curious how to bring this up to her. This morning when we woke up I thought she said she was taking me to breakfast. We went to this diner and she just left the check there so I took care of it. The last five times I have paid. She will always encourage me to eat what I want in the kitchen. She will cook for me at times. However paid for a large Kava Kava tab last night. I have taken her to meals a lot over the last couple weeks. She has very little money. However I do not like that I am paying for so much more than her. Yes she has the to little kids....fine. However I just think it would be nice if she offered to pay for a tip or something. Last night she actually did not thank me. It was 30 dollars for each of us. How do I bring this up to her? Yes I love her, but this is getting irritating, I know she has two kids... they are ages 4 and 6. I am curious specifically how to bring this up with her. Thank you so much for your time I think she is mostly oblivious.... I know to have two kids is expensive, but I think she maybe taking advantage a little. Yes we love each other very much. How do I bring it up? What should I say?