The idea that "those games were designed for children" is nonsense to begin with.
It certainly isn't.
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.