I would recommend to reconsider:

1. children() method that returns plain list - the modification of this list could lead to an inconsistent structure (for example if someone adds an item to it without setting parent of the item), so maybe it should be a constant method returning constant link? (all manipulations would be made with additional methods)
2. Maybe elements_vector should be changed from std::vector to std::list - there are many use cases that add, iterate or remove items from it, but I see no cases that uses items indexes. Adding (or removing) to the middle of a vector is not quite a cheap thing...