Create a multi-choice pulldown in a child record showing other related records

I have a unique case I can't figure out:

- I have Parent records and Child records

- On an Add or Edit form for the Client records, I want to be able to show the user a multi-choice pull-down that includes references to all of the existing (if any) Child records for this Parent.

- I'm not trying to do any Knack function after that - I'll use JS to do an action based on the selection made.

At first I thought I could do this by creating a one-to-many relationship between the Child and the Child again (self-referenced). Tried this, but couldn't filter the records to show only the records related to the Parent.

Any ideas?