Hi there.
If I understand your description correctly, I would say that you can use the “Create JSON” module before the API Call. In this module, you would need to define the data structure that fits the “children” structure. (You can actually use the data structure generator and generate the data structure by using a sample JSON code. Then, with the correct data structure, you should be able to map the desired values into the JSON module. This module then creates the JSON string for you and you can map this string into the “Body” field of the Make an API Call module.
Hope this helps.
Cheers,