i'm constantly terrified that my 2nd grader is behind. then she'll say or do something that assures me we are on track, lol. recently, i heard her explain to her 5th grade cousin (who attends public school) the difference between a monocot and dicot.
i've found a lot of sites that will help you figure out whether they are where they need to be. i usually just google.
There is a book called what your child needs to know in ____ grade. They have different books for each grade. At this point in their schooling they are not behind because they learn more from playing than doing school. If you are not having fun they will not like school. For those ages school should be fun they should be learning with out knowing they are. Playing games (candy land, go fish, uno, among others all teach valuable lessons that are easier to learn playing than in a book. My friend told me to use duct tape on the floor do different shapes and sizes of the shapes and ten play like simon says. Simon says go to a small square, go to a big circle." etc She also told me to put a little flour or corn meal in a dish with a cover and let them "write" in it. I did this with a boy I was babysitting not too long ago and he loved it. then he used it to make roads for his match box cars. when he was I put the lid on for the next time he came over.
I have 6 kids % have only been HSed and the youngest is 8 and in 3rd grade.