It could be argued that garbage collection, polymorphism, lambdas, and REPLs were semi-mainstreamed by Smalltalk. That's not to say that it was the first appearance of those features. However, it was a big incursion into mainstream awareness. In 2019, all of those things are squarely mainstream. In the 1990's, lots of programmers regarded them akin to "woo."
It was explored briefly in the early '90s at IBM (Hi, OS/2 Workplace Shell!), but the general consensus before the project was cancelled was to go with C/C++. I suppose education also saw some use, but various Lisps have most, if not all, of the above features and much more mainstream use in the '80s.
This is the first time I've heard that, do you have a source?
This is very widely known inside the language community and beyond. It's mentioned in many keynotes and talks by Alan Kay and others. You can go to the Wikipedia page and search for "children"
In one keynote talk, Alan Kay stated that the design goal was to create a common programming substrate usable by everyone, from children, to hobbyists, to enterprise programmers, to researchers. Arguably, they succeeded.
