Function children 

pub fn children(tree: &impl TreeStore, pid: u32) -> Vec<u32>

Get direct children of a PID.

use proc_tree::{DefaultTree, children, TreeStore, PidNode};

let tree = DefaultTree::new(100, 0);
tree.insert_node(1, PidNode { ppid: 0, cmd: "init".into() });
tree.insert_node(100, PidNode { ppid: 1, cmd: "a".into() });
tree.insert_node(200, PidNode { ppid: 1, cmd: "b".into() });
tree.insert_node(300, PidNode { ppid: 100, cmd: "c".into() });

let mut kids = children(&tree, 1);
kids.sort();
assert_eq!(kids, vec![100, 200]);
assert_eq!(children(&tree, 100), vec![300]);
assert!(children(&tree, 999).is_empty());