Here are clips from Sesame Street episodes that talk about breastfeeding…
Buffy breastfeeding Cody, from 1977:
Maria breastfeeding Gabriela, from 1988:
Moving away from Sesame Street for a moment, Mr. Rogers also talked about breastfeeding in Episode 1536 from 1984:
This song by Joe Raposo from the mid-1980s also shows a breastfeeding baby. Apparently the song has been updated and the breastfeeding clip was replaced with a baby being bottle-fed.
Nursing mammals from Sesame Street:
Enjoy!