The associative property of arithmetic is in fact introduced to your child at a very early age - but they just see it as adding numbers! And they are right - that is all the associative property is - but it is very powerful!
Until one starts to deal with variables, and rearranging mathematical equations, the power of this property of arithmetic remains relatively unknown.
The associative property of real numbers is the arithmetic property associated with the grouping of numbers. As stated above your elementary math student has already used this.
2 + 1 = 3 (add the 2 and the 1 first to get 3)
3 + 5 = 8 (add first answer (3) to remaining 5 to get 8)
Now let's change the order.
1 + 5 = 6 (add the 1 and the 5 first to get 6)
6 + 2 = 8 (add first answer (6) to remaining 2 to get 8)
The operations that behave this way are: Addition and Subtraction.