There you go:
$str = '<div>
<h1>Hello</h1>
<figure id="XXX">
<div class="abc">ABC</div>
<div class="qwe">QWE</div>
<div class="zxc">ZXC</div>
</figure>
</div>';
$html = str_get_html($str);
$str = $html->find('figure[id=XXX]',0)->children(1)->plaintext;
echo($str);
find returns elements and you need single element so set index 0 (the first one) at the second parameter in find