We drain our energy through thought process. These children are seldom "totally" lost in thought. They are admirably closer to the 'present' than we can ever imagine. They seldom miss to take notice of things happening around. They lack communication skills to describe all that they notice.
Now, this thought process, it drains energy, and at the same time, prevents the body to draw energy! We think that food is the only source, and stomach the only battery charger. When we are too happy, we do not feel that hunger so badly! When we are awe-fully excited, due to something awesome, we are closer to the present, and less lost in thought process.
You gave that example of children during train journey. There is a still more irritating example. Please look at a small baby which is yet to turn over to lie down on its stomach and lift its head a little (that small in age). All it can do during its waking hours is to cycle its legs in air, and put up its arms with fist closed, punching in air. It does this activity most of the time. The only intake is a few millilitres of its mother's milk (not litres/gallons). Now try to imagine ourselves lying down beside that baby and imitating the same physical gestures, for equal length of time. We too can sleep when the baby sleeps. We can eat all the calorie rich food available all over the globe, to be able to store necessary energy. This baby draws energy all the time, through air, water, light, etc. The skin is clean and tender, it breathes fresh oxygen rich air (ancient elders, doctors, etc advise only loose clothing for a tender baby). This child seldom engages in any thought process.
(Traditionally, in ancient Indian 'way of life', those who step into spiritual path are put on practices that helps a person to drop dramatically on food needs for energy. Dependence is thus bare minimum, on others, making that precious time available for gainful meditative practices. Such people eat just a fist full, once in several days, and sustain quite energetically on just water, air, and light. Eyes are input device for energy, mainly, not just for 'seeing' as we commonly 'see' ! It can also be used to deliver energy. We have stories, myths about graceful-glances, etc.)