Brady Bunch, The - Bobby lipsinking
In the scene where the kids are singing on the actual show, about halfway through the song, (when the camera focuses on just the 3 boys) watch Bobby. Just before the camera focuses on all of the kids again, you can see Bobby (when he is supposed to be holding out a note) keep his mouth open in a wide "O" and move his tongue around. That is proof that they are lip-sinking!!
watch the episode "Ametuer Night"