A Little Bit of Everything = A Whole Lot of Nothing

Surely we've heard this truth somewhere, from someone. But recently, we've seen it played out in the development of our son. Children will grow in stages, often focusing on mastering one skill at a time. It may even seem like they digress from already learned skills, while mastering the one in focus. Why then, in our education system, do we give students a little bit of everything all the time? Is there an age at which humans learn better by getting bits and pieces of topics over years of instruction? Surely, this has been thought of and discussed before. So where's the information? Starting search now.