How do I add a map to a type map field in Firebase using flutter?

Here is the code in the image
FirebaseFirestore.instance.collection('userNames').doc(docID).set(
{
'children': {
'childUserName': username.text,
'childPassword': password.text,
'childFirstName': firstName.text,
'childLastName': lastName.text,
'username': username.text,
'parentUID': currentUID,
'parentFirstName': parentFirstName,
'parentLastName': parentLastName,
'points': startingAmount.text,
'timestamp': DateTime.now().millisecondsSinceEpoch,
}
},
SetOptions(merge: true),
)
