Best way to do a query with an entity's children

[CoderJo-Pro]

I've been doing

for children in &q_parent {
    for child in children {
        if let Ok(components) = q_child.get_mut(*e) {
            // ...
        }
    }
}

later I tried

for (mut plaza, children) in &mut q_plaza {
    let mut iter = q_unit.iter_many_mut(children);
    while let Some(mut unit) = iter.fetch_next() {
        // ...
    }
}

But the iter_many_mut returns a QueryManyIter that has a fetch_next function, but only QueryManyIter that has ReadOnlyQueryData implements Iterator, and the ordinary QueryData don't.

That's hard to decide, they seem to be pretty inefficient. Did I miss something?