[`_.reduce`][1] would work nicely here:

```javascript
return this.make(_.reduce(this, function(all, el) {
  return all.concat(el.children);
}, []));
```

The ever-vigilant @fb55 will mark me down for not reusing one array, so here's an option which does so:

```javascript
return this.make(_.reduce(this, function(all, el) {
  all.push.apply(all, el.children);
  return all;
}, []));
```

I'm not familiar with the V8 optimization mentioned above. I'm inclined to avoid the following, as the unnecessary assignment would confuse future readers:

```javascript
return this.make(_.reduce(this, function(all, el) {
  return all = all.concat(el.children);
}, []));
```


[1]: http://underscorejs.org/#reduce