[Scheme / Racket ] Struct design decision: store references to parent and child structs within, or in separate structure.

Hello,

I'm working on a very simple to-do program wherein I'm defining tasks. Tasks can have several parent- and child-tasks. From my experience with SQL, whenever I've needed to express a many-to-many relationship, I've created an "association table." Would defining a separate structure be the best way of representing these type of associations in Scheme was well?