Mum's the word. I'm going to unlock their medicine cabinet while they're asleep.
That doesn't sound like a nice thing to do. What is the reason behind you wanting to see what's in their medicine cabinet?
I'm going to steal their medicine and then drink it in order to have a good sleep.
You need a good sleep at night and want to take medicine for it, but taking your parents without their knowledge is wrong. You could drink the wrong dose for your age and then have to be sent to the hospital, have your stomach pumped and then possibly they might face charges. I think you should just sleep on your own the normal way.
But I'm having trouble sleeping.
If you have trouble sleeping, see a doctor, but don't steal your parents meds. It's a betrayal of trust.