When our oldest was 3yrs old, he was very well-behaved and followed whatever rules we presented him with. Unfortunately, his younger sister is not so compliant. More than once, my husband and I have caught her unlocking the dead bolt and turning the door knob to both the front and back doors. Even after we told her that it was against the rules, she still did it again.
I am reluctant to put anything on the door handles that would make it difficult for us to get out of the house quickly during an emergency. On the same note, we have considered adding another dead bolt or a door chain up high, but are nervous that the children would not be able to get out of the house on their own if there was a fire. I suppose a compromise would be to add a lock that was out of the twins' reach, but low enough for our 6yr old to reach. By the way, this is against our realtor's advice because he says it will be more difficult to sell our home if potential buyers get the impression that this is not a safe neighborhood.