How to "tone down" dnd

My brother and I play dnd every week, and our younger sister (10) has started to feel left out. My solution to this is to create a character for her and run a one shot so she can play too. The issues are: 1. She is very easily frightened and worried that fighting will be too scary 2. I've never dmed before and I have no idea what I'm doing.

If anyone has tips on how to make dnd fun for an easily scared little girl that are easy enough to implant for a first time dm, I'd be very thankful