DD learned them right after she turned 8, and DS5 somehow learned almost all of them at the same time (incidental learning? osmosis?). DD has an only slightly above-average memory (if you believe the WISC) and DS seems to have an excellent memory (not yet tested, my observation). DS also seems more mathy than DD (although DD is good at math).

DD also detests rote learning - but she got through it and has been making excellent progress in math since then.

We used Xtra Math - it is simple, not entertaining, but effective for building automaticity of simple math facts.