Get a copy of the key, hide it. When she locks one of you out, shame her, then tell her where the key is. Afterwards, hide the key in a different location so that when she does it again, the key isn't there for her and she'll have to call you again. Repeat the process until she doesn't do it any more.
How the F is she locking the keys in the house anyway? I cant lock my house without a key after walking outside.
Ive had a Schlage electronic lock on my back door for 6 years now. Other than occasionally being iffy when it is really cold (-30°F or lower) it has been a great purchase. If I thought baddies would actually stop and try to unlock the lock before kicking in a window or the other door at the house I might have wanted something with a 6 digit pin but this lock is mostly so I would actually lock the door in the first place.