Preferred arrow directions in models of composite objects

Arrow in models of composite systems tend to have the oppsite directions in programming languages (both OO & Functional) and relational models. In programing landuages the parent knows about its children while in relational models the children tend to know about their parent.
There are several reasons for this
- relational models do not support multivalued attributes
- it makes it easy to grow a tree nondestructively in the relational model
- relational models have indexes andbult in operations like join that make it possible to find all the descendants of a node even when the arrows point upwards.

What is the preferred way in Catlab?
- parent has a collection of children
- each child has a parent