What happens to an existing separation agreement's parenting terms if a society becomes involved with the family?

An existing separation agreement's parenting terms don't automatically get cancelled just because a children's aid society becomes involved with the family - the agreement remains a valid, binding document between the parents unless and until it's changed by a new agreement or a court order. That said, society involvement is exactly the kind of significant development that can justify one or both parents revisiting the agreement's terms, since the underlying assumptions about each parent's home and the child's needs may have changed.

If the society's involvement leads to an actual placement change, a safety plan, or a protection finding, either parent can ask the family court to vary the separation agreement's parenting terms based on that material change, rather than assuming the original agreement simply stops applying on its own. In the meantime, don't unilaterally depart from the agreement's terms just because the society is involved - if you believe a change is needed urgently, that's a reason to bring the matter to court promptly, not to informally stop following the existing arrangement. A family lawyer can help assess whether, and how, to formally revisit the agreement given the specific involvement in your case.