You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: children.rs
    path of file: ./repos/pueue/pueue/src/daemon/task_handler
    
    the code of the file until where you have to start completion: use command_group::GroupChild;
use std::collections::BTreeMap;

/// This structure is needed to manage worker pools for groups.
/// It's a newtype pattern around a nested BTreeMap, which implements some convenience functions.
///
/// The datastructure represents the following data:
/// BTreeMap<group_na
    
    pub fn get_child_mut(&mut self, task_id: usize) -> Option<&mut GroupChild> {
        for pool in self.0.values_mut() {
            for (child_task_id, child) in pool.values_mut() {
                if child_task_id == &task_id {
                    return Some(child);
                }
            }
        }

        None
    }