How do children learn? Ideas we need to know for teaching young learners

Chances are that at some point in your teaching career, you will teach or will have taught children. I had the classic scenario of starting with adults and slowly but surely being given classes with younger and younger students, until one day I realised that I was almost entirely a young learner teacher. Thing is, I never got any clear guidance in all that time as to how that should have been affecting my teaching. How do children learn? Were there any learning theories I should know about?