With the picture of ballet students, I can't help but consider the performing lifespan of ballet dancer. Even with all their technique their body eventually cannot meet the technical demands, at 30+ their body is generally already showing signs of weakening. Piano is totally different in this respect, it is not our posture at the piano which is our main challenge, it is the transfer of energy from our fingertips to the keyboard.
How we effectively use our fingers, connected to the palm, the wrist, the forearm, the elbow, the shoulders, the back, your feet. I think the feet are so important, they ground you to the earth an give you stability, more important than the back posture. I can also close up my shoulders and still feel relaxed, you often have to do so when you play with crossed hand positions, if you maintained an open shoulder position while crossing you may indeed cause more tension.
The only indicator is relaxation, what does it mean to be relaxed, lie down on a bed with your hands at your side, this is relaxed! The next step is to sit at a piano with your fingers resting in the Chopin natural hand formation, then ask yourself what does this relaxation feel like. So when you play you must always return to this state whenever possible, maintaining deviations from this will maintains unnecessary tension. Yeah, sounds easy in words but it is the application while learning music that is the challenge.
A childs body does not have the muscle bulk of an adult and thus do not have to deal with as much. Also each and every child is different, some have good posture others have bad just as you get with adults so merely looking at a childs posture is no different than from looking at older people. But most children have good posture because they have not had time to develop bad habits. I know people close up their shoulders because of a body insecurity, I know some girls I have taught like this, some have physical problems that you have to deal with, so you have to treat each case carefully.