I have a 4yo and 6yo. The 4yo can add 2 to some numbers up to about 20. The 6yo can add any single digit to any number below 100. Most of the time. They both can read pretty well. We practice both math and reading every 'school' night for about 15 minutes.

The only goal is to familiarize them with doing stuff with numbers. The 6yo's kindergarten class has heavy emphasis on math, so I try not to get in the way of what they are doing. I avoid confusion by just staying away from the same stuff (at first, I thought I'd reinforce, but that turned out to be a bad idea. The 6yo needs a disconnect from what's happening in school, not a reminder. It's tiring).

The one thing I can confidently say is that what works changes as they pick up more concepts, and have more influences on learning (ie, other parent, school, other kids). My kids are not especially smart or gifted. The trick is that we stay just at the border of what they can do. We try new things, which fuels their confidence when they get it. We practice old things, which fuels their confidence with little cost. We stay focused and brief.

I try to get as many information and pedagogical tricks as I can. But they only work, if they work, for brief points in time. Kid thinking changes much faster than adult thinking.