The question here is what is happening? Some people might say that the kitten is comforting the puppy who is on a drip. I don’t think that that is what is happening.
What is happening, I suspect, is that the kitten needs a mother and he or she has chosen a puppy who smells nice and who has been chosen as his surrogate mother. It is the kitten who needs comforting. Although the puppy likes it too and therefore it is a win-win situation.
And what does a kitten do with a surrogate mother? He suckles out her nipple but what you see is a vestigial attempt (a remnant of what it supposed to be) to do suckle because all that’s left is kneading the puppy. As you may well know by now this is what newborn kittens do when they are being nursed at their mother’s breast to bring on the colostrum.
This is my assessment of what is happening but I would welcome the views of anybody else who might have a better idea. Please leave a comment.