Recently we taught the kids how to play Rummy and Phase 10. They're both similar games and the kids caught on quickly. Snow White still needs help, so we let her be on someone's "team". The beauty of these games is that at the end of the game you have to count up your points to figure your score. So we've got the younger kids learning to count by 5's and 10's and we've got the older ones doing double digit addition and subtraction quickly in their heads.
Daniel and I really want to focus more especially on teaching the kids to do "mental math" calculating quickly without paper. When we were kids we had Facts Master competitions where we had to fill out a math chart in less than a minute, usually doing one problem per second. Our kids need more of that. We're trying to think of what a good reward would be if they can complete the math chart for addition, subtraction, multiplication, and division in under five minutes. Meanwhile, we're having a lot of fun playing cards games!