It sounds awful but I'll explain...
Day to day life, I love his kids. They are great and we get along so well.
But there are times when I wish I wasn't at the bottom of the pile in terms of priority.
Yesterday I was in hospital for a medical management miscarriage. I had to deal with that alone because his children were with him. I was vomitting and in a lot of pain.
Today I'm emotionally and physically drained and all I want is my partner to hold me whilst I cry but his kids are here and so I have to pretend to be fine. He's taken them out so I can be alone but that in itself is hard. I don't want to be alone, I want him. Just to myself for a day 
His children should come first. I understand that and I accept that. But I'd be lying if I said there hasn't been times where I wished things were different. Where my partner could also be there for me in times like this etc...