Choose Parent And Return List Of Bottom Level Children?

I have a large flat hierarchy table. I am looking for a method to select a parent and return a list of my lowest level children. I am somewhat familiar with VBA but I don't know where to begin coding this.

This has been a great learning source over the years, but this is one of the few instances I cannot find my problem so I am now posting. Example below:

Child
Parent

4
5

3
4

2
3

1
5

40
50

30
40

20
30

10
50

22
100

If I select 5, I want values 2 & 1 returned.
If I select 50, I want values 20 & 10 returned.
If I select 100, the value 100 should return.
If I select 40, the value 20 should return.