How to make it so you can't take a picture on lockscreen on iphone 4?

I have a iphone 4 and i have a lock on my phone. And when its on the lock screen you can be able to take pictures while its locked. My lil sisters keep taking pictures on my phone and its annoying. Can you make it so you CAN't take a picture when your phone is lock on the lock screen. If you can… Could you please tell me?