There's a rather large and active market for all things retro gaming and I don't think it's children driving it. Many adults still find those games fun to play.
The idea that "those games were designed for children" is nonsense to begin with. They were designed with the limitations of the hardware at the time, that's why they are simple. But even back then it wasn't just children playing videogames and certainly most games weren't specifically "designed for children."
Ah, the old "Sorry for destroying society, I really knew all along that what we were doing was evil but hey, thanks for all the money! Bye!" disclosure. A classic among politicians and businessmen alike.
Perhaps you're confusing home computers of the time with consoles. Console games were designed for kids. That some adults also played them is not the point.
Home computers were a lot closer to the 'games by and for adults' experience of the mainframe/minicomputer paradigm.
Things were less clear in Japan, where adult-oriented games were released for consoles like the Famicom — as were kids' games with themes Nintendo management felt were too controversial for Americans to handle (e.g. Megami Tensei).
The closest thing to an adult game came with Mortal Kombat on the Genesis but even it was mostly played by young teenagers and tweens. If it had had nudity it would have been banned on both consoles.
Adults consuming kids' stuff and taking it seriously has been a big factor in Western console gaming for a long time. Chrono Trigger is a great example, as is Final Fantasy VII.