"I am NOT an only child!"

I was talking with Dayne yesterday. We were eating some ice cream and having a chat. He told me that a boy on a show that he watches is "an only kid" The conversation took a bit of an emotional turn after that. I SHOULD have kept my mouth shut, I don't know what happened, but for some reason I said "Well, you're an only child too, buddy" It clicked for me, right as the words poured out, that he shouldn't be an only child right now. I didn't correct it, I felt awful, but I didn't think that Dayne had caught it and I didn't want to upset him.
He kept it going though, by responding to me with "I am NOT an only child!" I knew that he said that because he didn't really know what an only child was. So, for the first time I did something that was extremely painful for me, I excluded Alexandra. I didn't know how else to end the conversation and I wanted to cry just allowing the words to leave my mouth "An only child is someone that doesn't have any brothers or sisters"
Icy cold guilt rushed through my veins. I instantly felt like a horrible mother to both Dayne and Alexandra. My brain went numb and stopped helping me to figure out how to correct this conversation. Dayne became the hero of our talk, when he replied "Oh! But I have an angel for a baby sister!" He had a HUGE grin on his face as he said it. He won and he was proud. He did what I couldn't.
When I explained the story to Steve, I told him that it's amazing the Dayne has such an uncomplicated understanding. It's not about how something is thought about "technically" it's about how it just is and Dayne gets that. It seems to me that no matter what, I can always get the most honest answer from Dayne. He says things with truth and without the complication of worrying how other people view things, what other people think or how other people will perceive him, he just is.
I'm going to try to take a note from the Dayne and just be honest in the most simplistic sense...perhaps that will save me some guilt in the future.

And from Alexandra: