Apologies if this has been posted before, but I just stumbled on this: http://www.math-aids.com/

They are "dynamically created" worksheets, meaning that you can generate new ones of a particular type. What I like about these is that, at least at the K/1st level, they have a lot of variety, so Hanni can try a sheet of this, a sheet of that (coins, tally marks, number line, skip counting) to mix in with the straighforward addition and subtraction problems.