I remember being taught with "math manipulatives"...the result was that I intuitively grasp the relationship between numbers. Maybe that would let you physically show your niece what "eight sevens" actually looks like (and the similarity to "seven eights").
edit: something like this, but with each number 1-10 as its own block and a different color. http://www5.esc13.net/thescoop/insight/files/2012/08/MaryMat...