If you need a "quick" way to help your children understand the "basic elements" of education, without going back and "re-doing" all the lessons they ....ummmmm well...... missed in k-3rd grade, wait until the child is about 10 years old and can read on his or her own.
Have the child READ through a children's reading encyclopedia that covers many different themes and ideas that are basic to education as a whole. Foundational things basically.
Here is a great example:
Have the child read through each story, write a few sentences about the story he or she read, and then test the child's knowledge.
Do not worry, a child, so long as they are not severely disabled CAN learn to read and understand basic education things.
The more difficult thing actually comes from teaching the child to HAVE a good attitude when it comes to the basics of life......hence, I recommend regular bible study and practice!
Note: Some of the older books and encyclopedias may contain "unproven evolutionary nonsense" or "racist ideas" or "culture misrepresentations" and so forth. This doesn't mean the books are entirely FALSE, its just that false ideas exist. Simply GLEAN what you believe will help the child understand the basics.