The presumed outcome for any student on Placement is that they will be successful unless there is clear and documented evidence that s/he is not meeting the required competencies. In reality, that the student is not demonstrating an understanding / awareness of Service-Users needs thorugh the Assessment /Planning process and by the application of theory.
There is a clear responsibility of Practice Teachers / Supervisors to provide regular Supervision (at least fortnightly) and on the student to demonstrate also they they have used the skills and knowledge of collegues within the Team.
In your case colleagues would know where Parenting Assessments were available and the processes for accessing them - in some Authorities because of resource implications that may involve a Resource Panel application.
It is a good 'rule of thumb' that where services are indicated that a Core Assessment is carried out and that the Analysis is carried out. A 'Child in Need' meeting should then be held involving parents and other relevant professionals and a Child in Need Plan drawn up - that identifies the areas of concern / need and how those needs will be met. It carries far more weight as it has involved other professionals and 'ownership' is clearly detailed.
Where children are deemed to be at risk there should also be consideration given to holding a Family Group Conference / Meeting as those frequently result in additional help from other extended family members being available.
Placement failure should only arise through clear Supervision / Guidance / mentoring not having been successful and there should be clear reasons and how that 'failure' camn be addressed. There should also be a clear Appeal procedure as the student is not always 'wrong.'
Hope these few comments help, and good luck.