My four and half years old boy has been learning subtraction from me.
Such as:

46 - 29 =
(30 - 20) + (16-9)= 17

Two days ago, I introduced him negative number concept.
such as: 3-5 = (-2) and 5+(-3)=2.

Last night, he showed me his own interesting way to deal with the same subtraction question without using the concept of borrowing:

46-29=
(40-20) + (6-9) = 20 +(-3) = 17

How about that.... smile