It doesn't matter how you create document. You would see children for content as long as there is default transformer available.
For example, if you upload an image, you should see something like this in node browser:
For this you would see 3 entries, one for original node and 2 for these childrens.
For contentless nodes, there will be no children and no additional entries in db.
Now, i was not able to clearly understand. But seems you are saying that when you upload a json file you only see 1 entry but for others you see 3.
If this is the case, then it is also expected, As JSON mimetype doesn't have any ootb transformer configured, hence thubnails won't be generated automatically. You would have to configure one yourself in case you are looking to have thumnails and stuffs generated automatically.
Have a look at this documentation for more details: https://docs.alfresco.com/5.2/references/dev-extension-points-content-transformer.html
(ACSCE, AWS SAA, Azure Admin)