This was a question asked recently at co-op.  My friend's question related more specifically to whether or not a child is ready to "move on" to the next grade level.   Here's the not-so-short answer I gave.  I'm sure my friend would enjoy reading your comments, too.

1.)  What grade you call a child is fairly meaningless as an overall rule. 

-Most children I know are working either somewhat above or below that particular grade in at least one subject. 

-If a child has struggled greatly in math, for instance, then go slower with math - starting now.  Who cares if the workbook says 2nd grade when they are actually in 3rd grade?  Who cares if they won't finish every workbook page by the end of the year if you go slower? 

-What's more important - that they understand a process or get through the book?  Also, is it more important to get multiplication eventually  - at their own pace - or to become so frustrated with "not getting it" before they're ready that they feel like a failure and never choose to like math from that point on? 

2.)  All schools - whether public, private or homeschools - have different expectations for each grade level.  In other words, what one school expects to be accomplished in 2nd grade might be an expectation for 4th graders in another school. 

-If your child is greatly struggling with an entire 3rd grade curriculum, you might research a different curriculum rather than blaming your child for not being smart enough to keep up. 

-Make sure the curriculum you're using is meeting their learning styles and even their interests.  I don't mean to say that every assignment has to be pure joy, but if your child is motivated by bright colors vs. black and white pages, find a curriculum with bright colors.  If they enjoy crafting over workbooks, look into hands-on curriculums.

3.)  I would never harm a child's self-esteem by telling them that they aren't ready to advance to the next grade level, or that they will redo all the same curriculum again next year because they "didn't get it" this year. 

-Whatever changes you need to make - redoing a particular book, buying a grade level lower, implementing reteaching strategies, taking things slower - should be adult decisions.  A child will be much more motivated to continuing trying and learning if they don't feel like a failure.

4.)  If you are drastically concerned:

-Talk to seasoned homeschoolers about the issues and get their advice.

-Look through books like What Your ___ Grader Needs To Know.  Remember that this is one person's opinion, though. 

-Test the child to see what level they are actually working on.  Remember that not every child tests well and the results may be a bit skewed. 

-Research the particular concerns on the internet to see what steps you might take.

-Get a tutor.

-Here is a website that offers some testing ideas   http://www.homeschool.about.com/od/teaching/Teaching_and_Testing.htm

Homeschooling is such an awesome opportunity for raising children who are bright, confident and allowed to blossom at their own pace.  God created every one of us to be different, unique and special.  Just because one of your children may struggle with a particular subject, or even all subjects, doesn't mean that God doesn't have great big plans for them anyway.  Help them to the best of your ability, while giving them an abundance of encouragement and love, and God will take care of the rest!  :)