So one adult was always giving bad advice and one was always giving good advice. But in half the games it was rigged so the child always found the toy even if they looked in the wrong places. The advice from the adults was still always bad and good advice for the rigged games.
Afterwards they asked the kids who they thought were more helpful. Even though one adult was giving bad advice always and one adult was giving good advice always the 4-5 years olds only discerned this if they actually lost the game when they didn't follow the good advice, even though the adult giving bad advice was always still giving bad advice.
I think the key is they must have rigged the game to ensure the bad advice adult was always still giving bad advice. If the bad advice turned into good advice in the rigged version then it wouldn't work, just like you said.