Everyone agrees the "best interests of the child" test trumps all in child protection proceedings. However, the innumerable child protection court cases which reveal five versions of children's best interests - the Children's Aid Society (CAS) version, the Office of the Children's Lawyer (OCL) version, the father's version, the mother's version, and the court's version - demonstrate the often highly subjective nature of children's best interest assessments. Child protection law is more "art" than "science." Which is why expert reports, while helpful, are never definitive in presenting the one ideal plan of care that will be in the children's best interests.
If one even looks at basic children's law principles like children are generally better off residing with their parents, parental contact should be maximized in access situations, and wishes of older children should be taken into account when making best interests assessments, competing views of best interests quickly turn the conversation into a quagmire if CAS insists neither parent is fit, both the mother and father insist it is the other parent who is unfit, and the OCL expresses the children's views that they want to live equally with each parent.
The demand you hear most often from child protection judges is for more evidence of best interests. Judges don't want to guess, they want to decide on facts. So if you - whoever you are - are putting forward a particular plan of care for a child, you're going to need some cold, hard facts to back up why that plan is feasible, rather than just wishful thinking.
Rules of evidentiary admissibility are pretty loose in child protection, so you don't need to get too hung up on legal formality. Letters written by relatives, or social workers, or medical professionals, or addiction counsellors can all work, though usually they should be appended as exhibits to someone's affidavit sworn to present the overall version of the facts. But each of those people don't need to file their own affidavits, and usually would not be required to attend court to testify. Getting your own expert witness would be best of all, but don't make the mistake of hiring an expert, and then rejecting his findings.
Showing up in court asserting your rights as a father or mother, arguing that a particular plan is in your children's best interests, and having no evidence whatsoever to back you up other than your own promises, usually isn't going to cut in in the face of conflicting CAS sworn evidence.
As a lawyer who represents fathers and mothers in child protection proceedings, I firmly advocate for my client's rights, and their views of what is in their children's best interests. However, my clients need to give me evidence that permits me to sell the court on the correctness of their arguments. So as soon you as father or mother learn that CAS is showing interest in your family, you should start compiling evidence that will assist you and your children in court much later.