Simple past or present perfect?

Simple past or present perfect? Would one of them be more natural to use here or are they practically interchangeable?

Example: A young man has hit a woman with his car. He immediately calls his mom.

Son: Mom, I/I’ve just hit a woman with my car. I think she’s dead.

Dad: What’s wrong?

Mom: It’s Ben. He/He’s just killed a woman.

Dad: He did what?/He has done what?