What's the easiest way to test a child stream so that it only fires and emits values for a single instance of the parent stream.?

A child stream will be synced for each child context generated by its parent stream. The parent stream usually generates 1 or more contexts for each record. Do you need a different behavior?

I wanted to test the output of the child stream, with one context instance from the parent. Other than changing the code of the stream to override returning more than one parent record, is there a (built in) way to test a child with a single context value during development?

No I'm afraid there isn't a built in way to do that. You could try writing a script that imports all the necessary components (tap and stream class at least), instantiates the tap, instantiates the child stream class, and then call

stream.sync(context=<YOUR CUSTOM CONTEXT>)

. But by all means since there isn't a built in way of doing this, do log an issue 🙏