When you say, "When you feel seasick, that’s your brain getting a little confused!", the sentence is mostly correct and easy to understand for an 8-year-old. But to make it a little clearer and grammatically perfect, you can say: "When you feel seasick, it means your brain is getting a little confused!"
This sentence explains that seasickness happens because your brain doesn’t quite understand what’s going on when you are on a moving boat or ship. Your inner ears, eyes, and body send mixed signals to your brain, and that causes discomfort called seasickness.