Its pretty common for 4 year olds to not yet know about planetary orbits, and to also find rocket launches fascinating

Don't be bamboozled! This is an amazing time to be alive!

I'm not seeing cues of satire, it all seems sincere.

...Huh? Why would that be sad?

My parents have formative memories of watching the moon landings as kids, and I of watching the space shuttle. As someone who ended up where I am in large part due to my curiosity about the world growing up, reading of OP's kid watching a rocket launch, thinking critically about what they're seeing, and learning more about the world from it is a joyous thing.

What is sad about this?

He's 4

what is the sad part here?

Relevant xkcd https://xkcd.com/1053/

I expect many kids to know that from cartoons, but perhaps my kids whatch weird cartoons. Anyway, the cartoons sometimes have the details wrong, so it's nice to talk about that.

Why would it be satire? We all just saw one of the greatest achievements in the history of engineering and we can’t feel good about it?