Interestingly, my DD was using an online application called First in Math in 1st and 2nd grade through her school. It was pretty good because the 'higher stuff' was open too - it held her attention until she found that 'Grand Master' was as high as one could go LOL
By the end of the second term of 2nd grade she had completed the 4th grade stuff there. One of the area and perimeter exercises involved working out the lengths of the sides of a rectangle with a perimeter of say x and an area of say y. I was impressed because pretty well the same reasoning lies behind the factorization of simple quadratics.
The school stopped that and we discovered AoPS but I have to say my DD enjoyed that app for a while. It really helped her with her fluency and mental arithmetic too.
