Reddit reviews Chester Raccoon and the Acorn Full of Memories (The Kissing Hand Series)

I'm so sorry for your family's loss. May I ask how old your niece was? If she was close in age to your daughter, that may be extra scary for her. It's much harder for kids to process a death when it breaks the neat tidy narrative of "when people get very old their bodies stop working and they die."

I would focus on how rare car accidents are, and how most of the time you can prevent them by driving safely. Help her think about the many, many times she has ridden in a car and nothing bad has happened. Also remind her that even when car accidents do happen, most of the time they just hurt the car, not the people inside it. Show her safety features on the car, like the airbags and seatbelts and her car seat. Reassure her that you will do everything you possibly can to keep her safe.

Also, continue to validate her feelings and help her grieve - even though she didn't know her cousin well, it's still a sad and scary experience for her to process. Most of the children's books out there about death are focused on the death of a pet or an older relative, so there aren't many I can recommend for this situation, but you might try Chester Raccoon and the Acorn Full of Memories in which Chester's friend from school dies in an unspecified accident and the other students remember him. It's more focused on sadness than feelings of anxiety, though, so may be less helpful if she's just scared at this point. You could try more general books about worries such as Is a Worry Worrying You?, When Worry Takes Hold, or Jonathan James and the What If Monster.

Chester Raccoon and the Acorn Full of Memories is a good choice.