I have this problem, too. My 3 year old keeps playing with my blackberries and making emergency calls. The 3rd time was yesterday and it is embarrassing and potentially costly.
Suddenly I understand why you can't disable this feature. When the phone is locked, you can't dial 9-1-1. When you press the keys, it starts entering your password.
Regardless, I need to do a better job keeping the phone out of my kid's hands.
One more thing I noticed about the locked mode is that it can change the text mode (i.e., number/symbols or all caps) and I end up putting in my password 2 or 3 times.
***Updated to add***
I started putting the phone in Standby mode, and it works much better! My little one got the phone this morning and he kept saying "it's off, it's off!"