Why is he inviting some kids to his birthday party, but not others? 

When you’re a childless adult in your 40s, you start to have a hard time keeping up with your friends who have kids. Often, they are preoccupied with their children and their activities, especially in the first couple of years of their kids’ lives. Parents might not be as interested in the leisure activities that were important to them before they had kids. The world of partying every weekend and throwing elaborate birthday celebrations for non-milestone adult birthdays is a thing of the past for most parents. 

That’s why it must be difficult for the parents in this story to deal with their childless 40-year-old friend’s wild demands. He is throwing a murder mystery birthday party for himself at his friend’s house, but here’s the kicker. He’s excluding one of the kids from the hosting family from his party because this specific murder-mystery experience requires seven participants. Instead of making it an adults-only party like a normal childless 40-year-old would, he is letting the 10- and 11-year-old children of his friends participate, but not the 7- and 9-year-old kids. He clearly doesn’t care about the themes being inappropriate for their ages. If so, he wouldn’t have let any of the kids come! He could’ve made this a family party instead of a murder-mystery party, so everyone could be included, but he cares more about himself than he does about the kids in his life. I know it’s his birthday, but he could’ve used his noodle a bit more!