Create a custom object with nested child objects

Summary:

Hi,
In a custom page we're building we have a custom object, that object has 3 child objects, and one of those child objects also has a child object. All of the relations with these objects are of the form 1-many.
Now while testing the endpoint of the main custom object i discovered that we can create the main object along with its child objects in one request, with the json like this:
{
"employeeId": "12345",
"description": "Test Discipline",
"dChild1Collection": [
{
"code": "103"
},
{
"code": "102"
}
],
"dChild2Collection": [
{
"emplid": "99901"
},
{
"emplid": "99902"
}
],
"dChild3Collection": [
{
"comment": "test comment",
}
]
}

However when i also try to add the child of the child object in the create, i always get the response 400 error.