Inside the box

OK, I have a problem. Auralee, my 5 year old experiments on Taryn, my 20 mo old. Lately she has been doing things that could possibly harm Taryn if not caught in time. The other day, Auralee put the toy bucket over Taryn and then put a bunch of heavy toys on top so she was trapped. She then went outside to play and left her all alone. If she hadn't started fussing a little I would have assumed she had fallen asleep somewhere and not found her for a while. Then, today, Auralee was playing dress-up. I heard Taryn's muffled cries and went to investigate. Taryn was locked inside the trunk the princess dresses are kept! No matter how many times I try to explain to Auralee what could happen, she doesn't get it. I don't know what to do.