Create Parent and Child Object Simultaneously with API

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I think my scenario is quite a common scenario: I have an external campaign that requires custom contact information to be saved in the database. I've run out of row size space to add any more custom fields or custom attributes to the Contact standard object (also it's not best practice) so I've decided to create a custom object called "X Profile" (X is the name of the campaign) which will contain the campaign fields and have a relationship to the Contact object through the Email Address field. I have a third party developer that will develop the application to collect the information