What do you call this cute pose that usually young babies adopt?
Learn English – this cute pose called in English
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Preface ADDED 14 Jan/Jan 14, 00:11 GMT/UTC: This answer, as the OP Lucian Sava is well aware, has been written from an AmE perspective. Actually, it is written from the perspective of the variety of AmE that I speak. I think Lucian gets by now that English usage is not universal, as I had already indicted in my anwer. Let the conversation continue, by all means!
Serpentine is fine, but in English I am pretty sure we would use it as an adjective describing the road. Such a road is also called, in familiar terms, a curvy mountain road. More technically, it is any road that contains many switchbacks or hairpin curves
Switchback can refer to entirety of such a road. But this may not reflect universal usage.
I think serpentine (adjective) would be the more universally recognized word.
You can do an image search for both "switchback road" and "serpentine road" and get many of the same images.
Thus saith the snake:
I think baby talk is the common and proper term. It refers to the words or sounds a baby makes when it's learning to talk. In addition, it also refers to special language adults sometimes use to talk to babies.
We also can call it babble. We can use babble both as a noun and a verb.
Best Answer
I would call it a {baby/kid/child} handstand or headstand. If I search Google images for those phrases, the image in your post comes up, as well as some other examples (click for larger size).
Handstand was the first thing I thought of, even though a real handstand would have the person's feet and head lifted off of the ground. A headstand has some of the weight of the person's body resting on the head, so in the image above where the child has their cheek on the floor, it looks more like a headstand than your example picture, which seems more like a baby handstand.
Another phrase would be "upside down baby" - That describes the content of the image, not exactly the pose.