What do you call this cute pose that usually young babies adopt?
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.
For babies and toddlers, it's usually called
looking between their legs
For adults it could be
looking between their legs
bending over and touching the ground
toe touching ( callisthenics )
Downward Dog ( yoga )
In Yoga this pose might be called a 'forward fold' or a 'standing forward bend'
(Also may be called: Padangusthasana or Uttanasana but that is not English)
