It's not sold anymore

It is, just not from the Apple store.

https://www.apple.com/iphone/compare/ "Available at authorized resellers"

Translation: "Discontinued, but 3rd-party retailers will keep selling their stock until it runs out."

So what's wrong with topbanana buying the SE for his kids right now?

For one, it won't be supported anywhere near as long as a device launched today. It would still be getting iOS updates during their entire high school career.

Nothing, but should probably move fast if the intent is to get a new one.