Basically, you want to loop through each direct descendent of the old-parent node, and move it to the new parent. Any children of a direct descendent will get moved with it.
var newParent = document.getElementById('new-parent');
var oldParent = document.getElementById('old-parent');
function move() {
while (oldParent.childNodes.length > 0) {
newParent.appendChild(oldParent.childNodes[0]);
}
}
#old-parent {
background-color: red;
}
#new-parent {
background-color: green;
}
<div id="old-parent">
<span>Foo</span>
<b>Bar</b> Hello World
</div>
<div id="new-parent"></div>
<br>
<button onclick="move()" id="button">Move childs</button>