I have found a couple of websites that I frequent for math enrichment.
They are from 2 text book companies which publish online support materials.
The first one is Houghton Mifflin Math. I use the table of contents from the leveled practice link to pinpoint appropriate material to support the concepts that we are working on. For example, if I'm helping my 5 year old with adding by counting on then unit 6 from grade 1 reinforces that concept.
By selecting the leveled practice link, we are able to view all the lessons in the unit and the type of worksheet to generate (practice, reteach, homework, problem solving and English learners.) The reteach is helpful if I'm looking for another way of explaining a concept (since Houghton Mifflin is not my primary math curriculum.) The .pdf files that support these units are quick and to the point. Click one circle from the 2 sections to view a worksheet.
Another good math resource is the Harcourt online games which correlate to their math program. These are a nice treat, or a good way to keep one child active while working one on one with another. The games are simple but fun for kids.