Update custom object relationships via the API

Right now it's not possible for me to add a related object to a custom object via the API, and I would like to be able to do so, so that (at some time after the creation of the adult's record) I can link the new records of minors to their parent's custom object.

This would also allow me to change the parent of a minor, e.g. in the case of an adoption/change of custody or guardianship. Right now, when I GET the info for the parent custom object, there is no way to see the anything about related records (which I've added manually) in the JSON response.

Please create this feature. From my understanding, the main purpose of custom objects is to allow for dynamic relationships to other objects, so it seems pretty weird to me that there isn't a way to create/edit the dynamic relationships of custom objects via the API.

Thanks.