And they grew the bitterness down over the years, too.

I hated raw broccoli as a kid, didn't "like" it until I had fully-cooked, soft broccoli, so I'd say your hypothesis is flawed.

I don’t know where I heard or read it, so take my explanation with a grain of salt. Most poisons taste bitter, so avoiding bitter tasting things means avoiding poisonous things. Since children have a smaller body weight, the lethal dose is smaller for them. As go grow older, your body mass increases and so does your tolerance to poisons. But I am not sure why we would not want to avoid poisons in general even if they were not lethal any more.

The idea is that some kinds of food are hard to digest or otherwise problematic for kids, while adults can eat and digest a broader diet.

Maybe you never had those receptor cells. We are talking about 15000 different bitter receptors in kids vs. 5000 in an adult. If you were lacking one in 15k it's not much of a miss.

Having a kid who always ate broccoli since solid foods were introduced, I think it comes down to more of what options the kids have available, and what their taste buds expect.

A lot of "kid food" marketed to parents is sugar filled, processed shelf stable stuff. If you give your kid "fruit snacks" and Teddy Grams as part if their daily food, of course they're going to think broccoli or other veg is disgusting.

So even if they have "double the taste buds" (something that I've never seen or heard a doctor tell me in all these years), but even if that's true, if the palate is used to sugar/ultra-palatable foods, yeah, broccoli will taste like shit.