What to do

My boyfriend and I planned a date to a restaurant that iv been wanting to go to for a awhile. He had to work on my sisters birthday so he said he would take off the following Sunday and we could have a date. His kids left on Wednesday for vacation and weren’t suppose to be back till Monday. So perfect we won’t have kids on Sunday, he doesn’t have to go to work, we can go on our date right. He tells me on Thursday that he has to work on Sunday but he worked it out so he gets out early and we can still go. Like ok what ever were servers, you need to go to work to actually make money, understand. So now it’s Sunday he’s suppose to get off at work at 4 so we can go on our date. His ex wife calls him right as he walking out the door, they came back today. She’s on her way to drop the kids off. It’s cool I order a pizza my boyfriend leaves for work, the kids show up, we eat they tell me stories of Florida all is

Good. My boyfriends leaving work now and he calls and asks if we can move the date to tomorrow so we can have dinner with the kids. And I understand their his kids, he hasn’t seen them since Wednesday. But obviously I’m upset we set up this date a week ago. Point of this post is how do I not act upset when he gets home. Cuz he’ll get an attitude if he sees me with a sad face or a sad tone of voice.