I have three kids and they were all fantastic sleepers. In CB's shoes I could have kept them up late. I could have put them down at her house and taken them home and put them to bed. I still would have been ticked if I'd been invited for dinner at 6:30 and not served until almost 9. I would have had snacks for my kids but I would have been tired, hungry and grumpy! The child's sleep schedule is a red herring, it doesn't matter why CB had to be home by 8:30. Unless there are extenuating circumstances like your oven broke and there's nowhere else to get food, it's rude to delay dinner by 2.5 hours, especially if you're in the kitchen preparing it that entire time and don't bother to socialize with your guests, offer snacks and drinks, etc.
CB was not rude to leave when she said she needed to leave. It doesn't matter why she had to go. Her husband did the right thing and declined the invitation for 8pm, explaining that she needed to be headed home by 8 (or shortly thereafter). Her hostess acknowledged that and chose to change the invitation to accommodate CB's need to be home at 8. The hostess then delayed dinner, either through deliberate snottiness or accidentally due to lack of planning, forcing CB to have to leave before dinner. There was nothing rude about CB having to leave.
Let me change the scenario a bit and share something that happened to me. I needed a consultation with a new doctor. I called the office and was offered an appointment for 3pm. I explained that I couldn't make that because I had another appointment that couldn't be changed at 4pm and it was half an hour away, so I needed to leave the office by no later than 3:15 to be safe. She said she understood and offered me an appointment for 2pm and assured me that they keep a tight schedule and I'd be out of there in plenty of time.
I show up at 1:45, check in, and wait... and wait... and wait. At 2:45 I went to the counter and asked if I would be called soon, explaining that I had to leave by 3:15. I was told it wouldn't be much longer. At 3:15 I gathered my purse and jacket, went back to the reception counter and explained that I had to leave and why. I also told her I expected my co-pay to be refunded. Was I rude for leaving? Nope. I explained, twice, that I needed to leave by a certain time. If they thought I was bluffing that was their mistake.
(As it turned out the doctor was fantastic, and really was generally on schedule, but there'd been an emergency with a pregnant patient. He called me on my way home and insisted I come back during his lunch hour on whatever day was convenient for me and was never late again for any of the appointments that followed.)