Just wanted to share a quick update.
Apparently TPTB at BK agreed with the view point that the manager should have at least made an attempt to speak to the mother. I actually received a phone call from a representative, which was unexpected.
She basically said that, regardless of the child's mental age, if she was physically too large to be on the play equipment, then it was a safety hazard for both her and the other children. And the manager should have spoken with the mother.