I have a taxonomy vocabulary with 2 parent terms with children (fishing, and hunting) I want to get all children of the fishing parent term. I've tried to do it with core, but not sure it is possible.
I'm not exactly sure what you need to implement, but here are the steps to get you what I think you need. For demonstration, I created my own vocabulary with animals like so:
Then I created a view that will accept any of the term names as a contextual filter and return its children like so:
If that's what you need, then here is how you create the view. I'm not sure how you wanted to display them, so I didn't create a page or block display.
Step 1: Add new view
Step 2: Add contextual filter for parent term
Step 3: Configure the contextual filter like so
Save that contextual filter to the view and it will return the children based on the name of the term you pass to it. There may be additional steps to get it working the way you want it, but you weren't really specific. I hope that gets you started.