Corrected sentence: When you feel cold, you get goosebumps. Your body gets little bumps and your hairs stand on end. This can help keep you warm.
Explanation for a 6-year-old — step by step:
- When your skin feels cold, tiny muscles make little bumps called goosebumps.
- At the same time, the hairs on your skin stand up a bit.
- Those bumps and hairs trap a thin layer of warm air next to your skin.
- That warm air helps your body stay a little warmer.
Your original idea was correct. I changed the punctuation and split it into short sentences so it reads more smoothly for a young child.