Most of the questions on MSE about educational resources are for high school. At least I haven't found questions which deals with sites on very elementary mathematics.
I will be grateful if someone provides me with list of educational resources for kids - e.g. of the level of kindergarten, primary school and junior high school.
If you think that this question is not for MSE, tell me were is it appropriate to ask?
On
Though not a direct answer to your question, another resource you may be able to use is a university sponsored Math Circle program. They really are amazing and I am extremely jealous of those kids (I wish I got to go to a Math Circle when I was their age!). If you're not familiar with what it is, it's basically a program where your kids can go and learn mathematics they wouldn't normally be exposed to in school, with peers of their age.
To give you an example, at UCLA (where I attend) the fifth grade age group learned basic abstract algebra, cryptography, and fundamentals of proofs.
Check your local schools out! Math Circle Main Website
Here are some things I found in a few minutes. I'm also including what seemed to be the best google search among those I tried.
Google search for “elementary school” mathematics resources links
Math Archives: K-12 Teaching Materials
Awesome Library: Elementary School Math
Ask Dr. Math: Elementary Archive
Ask Dr. Math: Middle School Archive
edHelper.com: Math Worksheets and Math Printables
FunBrain.com’s collection of math games