My partner told me I will always come last to his children? Am I selfish for asking equal treatment?

So I am dating and luving with him for 6 months. For 5 months the mother of his last child refused to allow him to his daughter and son (not his she had him from prev relationship but he looked after him too since he was very small) but since 3 weeks suddenly she decided to allow him to see them on Sundays. To note that Sundays were the only day that he didnot work and that we had to spend together as a couple and to things we can't do on weekdays.

I am so happy that he can see his children because I know he was really sad but now on Sundays when his children come he completely monopolizes his attention to them. Like he will not even ask me to come and sit with them or you know like making me feel welcomed. I feel like a stranger when his children are here. We used to visit our family on weekends from time to time but now he says he don't want to go anymore and tells me to go alone. I told him that I know his children are important but it does not have to always be Sundays. It could be any other afternoon since even on Sundays they come on afternoons. I don't mind him giving them anyhting but what bothers me is that he had not bought groceries for the house for the past three weeks but just bought 2 bags of groceries for them. He bought them expensives things while I got everything cheap. I don't mind cheap things actually I don't even care but if I believe you should show love equally. I mean he cannot buy something for me and his children but what he gives his children is 4 times more expensive. And what makes me angry is that his ex had told him she didn't need his help but now she keeps asking for things. I feel so bad that I am jealous of kids but I actually just want to feel like I am important and that I am not just getting leftovers. When I talked to him he told me he has 4 kids and I have to accept I will always come last whether I like it or not and if I don't then I can leave. I don't want to be first over them I just want to be equal.

My partner told me I will always come last to his children? Am I selfish for asking equal treatment?
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