My recommendation for K is OPGTR, Singapore Essential Math B and HWT K - plus the library for great literature (check FIAR, Sonlight etc... for booklists) as well as non-fiction books on their favorite topics.
I also like the R+S workbooks, but the whole set was overkill even for my workbook-loving child. For my other kids I will probably get books C and G for the math teaching and book F for the cut and paste skills.