Additional functionality You haven't indicated if/how your users are able to change order of items, and indentation, something that's frequently necessary in my experience.
Consider Alternative: use a multiline text field with indents, and a side-by-side-preview (similar to stackexchange)
For experienced / frequent users this is significantly faster. Discoverability needs to be aided, e.g. with a pre-filled sample for new outlines, and/or the "Outline cliff notes" near the input.
This is not the best solution for infrequent users, however. Moving around items is darn easy for keyboard jockeys, however, especially novice users will end up using the mouse which requires good aim for selecting exactly an entire row. Your call.
If possible, allow to add multiple items for one roundtrip, e.g. a "one child item per line" text box. The respective "details" form" for more complex operations (like adding a node, picking a link target) can be loaded in an iframe. For moving items, you'd need to support cut, cop, paste so you don't need one roundtrip for each "move up once, move up one, ..."