How to test my grandkid online using parents validation without grandkids seeing the solutions?

I am a grandpa of 27 eager young grandkids spread across four states. I was terrified of math when I was a child, so I want to encourage my posterity to gain confidence in their math skills. I began a program to reward each of them $25 per unit if they would memorize their math facts in addition, subtraction, multiplication, and division, and combinations of the above. So, they could earn a total of $150 for completing all six units. Math facts have many online drill applications, and it was easy to come up with a program that the parents could validate and send me the results. It was a hit, and a majority of the little pups have earned their rewards.

But now some are beginning their higher math studies. I want to continue to encourage them, but “higher math” is a vastly larger challenge. Khan Academy is an excellent source for math testing, but with one small problem. I am unable to find a way to test them without also them having the ability to look at the answer before completing the test. I want their parents to validate their test results one “concept” at a time.

Can you suggest a way I can accomplish this? One concept at a time, one test at a time, and validation only by the parents?