Okay,,,try this scenario. She realizes that the top-load would not look like an accident as she wanted to the scene to appear that the kid climbed in on it's own. Therefore, she goes to the front loader, as it would be more apt to be able to be climbed into while mom was busy. Then she pretends to scream and rant because she cannot get the door open, when in fact she knew that nobody would be able to open the door...thus successfully killing the kid. She did not anticipate a policeman being able to break the glass with his stick, or at least not in time.
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