I was recently discussing accidentals with one of my students. She's 8 years old. I was explaining that an accidental stays the same through the entire measure and then changes back when the measure is completed at the barline. This was her response:
"...Just like when you have company! Everything changes while they're at your house, but changes back to normal when they leave!"
What a great explanation! I'd say she understood completely, wouldn't you?