I second the recommendations for Beast Academy and Singapore Math, although for summer practice I really love the Intensive Practice workbooks. I think they are more challenging and more fun than the regular workbooks.

These seem best to me for practicing keeping concepts fresh for a child who has got her memorized facts down pat.

For children who need to work on memorization and fact work, Kumon is superior. But my kids hate it.

I hate Dreambox. Doesn't seem like it provides enough practice to me, for the time devoted to it.

For summer, I have my kids use Dream Box. It is online. It is a little pricier than workbooks. But I found it easier to manage.

It also seems to align nicely with MCPS.

Singapore Math Common Core edition, available on their website. Don't be fooled by copies on Amazon, they are not the same.

For your 4/5th grader, I recommend Workbooks 4A and 4B.
For your 6th grader, Workbooks 5A and 5B are both a review of 5th grade as well as a primer on 6th grade. One caveat: the negative numbers they see in Math 6 are covered in Workbook 7A (they don't have a series 6, go figure). Workbooks are not in color inside.

The great thing is that these workbooks are based on the math curriculum of the country of Singapore, which has one of the best math test scores in the world. Most of the concepts are extended to greater depth than your typical MCPS classroom work.

The math level in the books is ahead of the MCPS curriculum, but the Common Core methods and progression are very similar. You might want to buy the corresponding Textbooks as well, which have the added appeal of being cute and colorful.

4th grade: Beast Academy

For your 6th grader I'd print off the summer math packet (on your school's website) and use that to make a selection from
B&N.

Kumon makes grade level books...if you go on amazon and search "kumon grade 4" you will see what is available. DK makes a workbook called "math made easy" for each grade--it covers many many topics, pretty thorough. You could also look at Singapore math workbooks for your child's grade--the problems in those books generally require more thought.

I am seeking advice on summer math workbooks appropriate. Maybe some math teachers or better-informed parents can help me? I did see a list of recommended websites, and I like IXL, but I think workbooks would be better.

Here are my kids' levels:

4th grader starting accelerated math 4/5. Is there a workbook that will help with the various concepts that will be covered so she can hit the ground running on those?

6th grader starting regular math. (6.0 I think it's called)

I found a lot of "common core" math books on amazon for grades 4, 5 and 6 but there are so many and even many with have good reviews. Anything specific to look for that's particular good for MCPS?

I know there are lots of kids for which math comes so naturally they don't need practice, but I think mine need it....