VARCHAR would probably not be a good idea here as the data is likely to exceed 255 characters (MySQL restriction) pretty quickly.
If the post-er wants to serialize, I'd imagine that blob or text would be a more flexible solution and without the 255 character restriction.
Personally, I'd go with a single table approach (assuming that each node has only a single parent) using a parent node id field...
| nodes |
| node_id (UNIQUE INT) |
| parent_node_id (INT) |
| data (whatever is relevant) |
That's just an off-the-cuff response, but as your nodes and subnodes theoretically contain the same information I wouldn't go sticking them in different tables. This way works by simply adding a parent_node_id column that would allow you look either up or down the node tree any arbitrary amount. It's not necessarily the fastest way (because of the multiple selects to move up or down the tree, so as the nodes reach 'n' levels they take, what is it, O(n) time) but it represents a relatively consistent interface that won't throw you into problems with your database.
A question of tradeoffs I suppose.
