We use Singapore Math with MFW, which recommends using skip counting verses using math facts. Skip counting has turned our daughter into a human calculator, and math is the one subject that does not come easily for her. Homeschool Creations Blog has posted these wonderful printable charts. They would be perfect for a workbox. Even a small child can learn to skip count by 2s and 10s.
I have not been able to find skip counting charts or cards to buy, and these are FREE and look professional. I have found using attractive tools increases their joy of learning.
UPDATE~ I found some typos on the number 4 chart, hopefully she has fixed them and has posted an updated version. So read all the charts before printing or print and correct by hand.