Doraemon's debut in 1969 has a close contemporary in a notable work of the manga artist Go Nagai, Harenchi Gakuen. In fact, Doraemon had debuted just one year afterwords! Gakuen kickstarted Go Nagai's infamous reputation as the man who introduced erotic content in manga, with lots of nude girls and guys who lusted after them. Still, the nudity was mostly reserved, and intended to be cute rather than truly sexual.
The manga was still going strong in the 80's, around the time another children's manga noted for use of female nudity, Miss Machiko was making waves. Doraemon and the rest of Fujiko's works would be published in their own magazine, CoroCoro. Rumiko Takahashi was also breaking out with a kind of children's sex comedy Urusei Yatsura, the first in her long career which would later include Ranma 1/2, a manga whose anime adaptation was notorious for being the one series [adult swim] would never run, even when they picked up the older-skewing Tenchi Muyo! series after chopping it up for a kid-friendly release on the Toonami block.
There's also the rise of Doujinshi culture in the 1970's, moving towards parodies of pop culture in the 80's. Doujins often involve adult material, and some of them even involve children, disturbingly enough. However, unlike the west where pornography can instantly destroy one's reputation, some artists like Kouta Hirano started as ero-manga artists before moving on to the big leagues. Notably, one doujin artist worked on another CoroCoro manga for children, a Pokemon manga called The Electric Tale of Pikachu, noted for having so much sexual content that the English release was censored heavily to remove a lot of it.
If there's anything to take from this, it's that sexuality is nothing new to children's manga, nor is nude women. But these manga also contain plenty of naked little boys, who often have full-frontal moments. But whether it be boys or girls, both examples of child nudity are mainly used for humor's sake. Take the example of the Hot Air Balloon; it involved both a boy and a girl in a shared embarrassing situation, and neither are particularly happy about it.
Doraemon concluded in 1996, but still remains popular even today. The character has been made a national icon, a sort of "emmisary to the world of anime". And part of that may have had to do with his breakout into the world of television a decade after his debut. Tune in next time, and I'll tell you all about it...