Always keep a record of the time you work. In addition, you arrangement with the parents should be written, so that it is clear what you get paid. This makes handling discrepancies much easier, because it is less of a personal thing and more of a business issue.
If there is a discrepancy, then say something like..."If you have a moment, I'd like to review some numbers with you. I'm sure you probably just calculated incorrectly, but I was paid X amount when I should have received Y amount for that time period. If you would like to review my figures, I have them right here." Then go over your hours and fees.
One of two things caused the discrepancies you've notice. Either A) your employer made a mistake and did not calculate correctly or B) your employer is shorting you on purpose. Neither one is acceptable.
To help with the awkwardness, try to picture yourself as a more famous entity that receives money for something and picture your employer as someone who pays for what you have. Let's say you are McDonald's and you just provided a hamburger to your employer. If they did not give you, McDonald's, the proper amount, you would bring it to their attention, right?
Or let's suppose your employer walked over to your purse as you watched, took out your wallet, and removed half the money. Wouldn't you say something?