I have a custom object Parent__c and a Master-Detail relationship with Child__c. I've been writing various InvocableMethods that use JSON.serialize() to send data to an external API (that I control, BTW). Works great. The invocable receives the object itself Parent__c, making the code really simple.
But it appears that the children are not serialized at all, which probably makes sense.
What is the best practice to do this? Any tips or ideas?