A hand made clay dam is easy to make and all that is needed for the first ricefield
Do you remember the youngest years as a kid, playing with clay or soil to create small ponds, dams or other simple structures ? A large rice terraces project is nothing else but the very same as many of us did during youngest childhood years - only a little more of the small units.
The very same applies in all situations of life. A 10 year old kid easily can build a wall of a few dozen bricks per day, adding another few dozen bricks each day. Or adding another meter or 2 to a clay dam in a rice field. Months after months until one complete unit done.
Hence every child can start and successfully accomplish such a rice field project or any other project in life. Just a little slower than a 16 or 18 years old or a 40 years old man. All have one in common, we all start piece by piece, step by step. If you divide your final project into smallest work units of several minutes or a quarter hour each unite, then even the largest project appears to be nothing more than a large number of small steps.