The real problem here is the nostalgia factor. A lot of people grew up having the planets ingrained into their brains with various mnemonics. Hard to say goodbye to “pizza”.
If Emilia is a scientist at heart and heard that Pluto was reclassified because we found many more like it, she'd probably be fascinated. Mind blown, even, that we've found Sedna, Ceres, Makemake, a bunch of others I've forgotten the name of, and a few more that just have a number.
On the other hand, she was just introduced to the concept of things like interstellar travel and aliens, so "someone took away one of our planets" wouldn't have been all that far-fetched. (Especially since the dialogue in the post doesn't actually mention Pluto. Maybe she thought it was Venus.)
If you want to include all the dwarf planets then we've got more than 9. Pluto was only the latest object to be called a planet and reclassified. No one complains about the others.
I really don't get why that would be a problem. Personally, I would be delighted if we had more than 9 planets. "Oh no! We can't have Pluto be a planet cuz then we'd have 5-10 more bodies that we'd have to call a planet too!" So what? Give me 15 planets; hell give me 50, that sounds so much cooler than reducing the number of bodies in the solar system people care about by 1.
Now I'm curious: How publicised was Pluto's discovery in 1930? Did the public care? Would Earhart likely have learnt about it before she vanished in 1937?
They were basically calling it the great American scientific discovery at the time. Which is probably why some people were so loath to accept the "demotion."
Pluto was a planet, once. The crown jewel of the Plutonian Empire. But the Plutonians got greedy. They dug too deep in search of plutonium and awoke a terrible horror. Now all that's left of Pluto is a cloud of asteroids and a celestial dwarf, barren and uninhabitable.
The problem with Pluto being a planet is that we would also have to classify something like thousands of other objects as planets as well. That's the whole reason it's not classified as one anymore.