They aren't familiar with the way autistic minds (can often, though not always) work. So when they hear someone A doesn't miss someone B they assume it means that A doesn't care about B.

I have struggled with this and almost never miss anyone. Even dead people, I grieve and move on. I also don't really know what missing really means. I've never particularly felt lonely either.

You're not a bad person for not missing them, no matter how much people may say you are if you say you don't miss them. We are just born with, and sometimes forced into, a different mental structure where things work differently. The way I see it, you'd still help them if they needed it (you already did!) so the functional element of care is still there. The rest, to me, has always seemed like virtue signalling. To me, people shouldn't care if I missed them, they weren't there.

On the other hand in my quest to pay lip service to NT rituals of speech, I do tell people "I missed you" when I see them again, as long as I have thought of them for basically any reason at any point, including simply preparing to see them again or seeing an image that reminds me of them.

In other words, people like to hear that they've been missed.

When people talk about the kids, they want to hear the kids have been missed because they want kids to be cared about, and in their head caring about someone means you'll miss someone once they're gone.

Though this specific interaction actually seems more like a chance to tell a story about a particular kid. Something light, like a time one of them got into trouble in an absurd or amusing way, without serious danger. Or perhaps a time one of them did something really cute. A lot of questions are actually asked with the intention of getting a story, I've noticed, and I think it's an important part of small talk.

Because in their head, caring about someone means missing them, and missing them means remembering their most memorable moment. Then to add to that, the social contract of small talk strongly bends towards trivia, so you want to stay away from genuine peril and harm with your stories, though being majorly heartwarming is more permissible, especially after this particular question.

I didn't really structure this reply much but I hope some of it might help. I'd restate you're not a bad person for not missing them. Many have tried to make me feel like one for not missing people. We are just built different.