Allow parent change on items that are not parents?

[jaradc]

Long time admirer of this amazing jQuery work of art. It's just amazing! Genius really.

Quick question for anyone: See below? The top item 0 is the root node. The rest are all parent items.
I'd like to allow child items to move freely between parent items, but not let parent items with depth of 1 be moved into another parent.

disableParentChange: true disables parents moving under another parent, yes. But, it doesn't allow for children to move from one parent to another.

0: {item_id: null, parent_id: null, depth: 0, left: 1, right: 16}
1: {item_id: '1', parent_id: null, depth: 1, left: 2, right: 3}
2: {item_id: '3', parent_id: null, depth: 1, left: 4, right: 5}
3: {item_id: '2', parent_id: null, depth: 1, left: 6, right: 7}
4: {item_id: '4', parent_id: null, depth: 1, left: 8, right: 9}
5: {item_id: '5', parent_id: null, depth: 1, left: 10, right: 11}
6: {item_id: '6', parent_id: null, depth: 1, left: 12, right: 13}
7: {item_id: '7', parent_id: null, depth: 1, left: 14, right: 15}
length: 8

How would I use the isAllowed function to say "If item has depth equal to 1, don't allow the move/drop.?