1. Navigate to Customization > Lists, Records, & Fields > Record Types > Click on a record set as child
2. Records are Ordered = unchecked
3. Allow Child Record Editing = unchecked
4. Click Save
Note: However the steps provided above will disable adding of child records in Edit mode directly from parent record.
More information about Parent/Child records can be found under SuiteAnswers article 10149: Using Child Records.