I need some tips for babysitting!!?

I am babysitting my twin cousins on Monday, Tuesday, Wednesday, and some Thursday for their grandma (my great aunt). Anyway, my main job is making sure the boy twin doesn't escape his room when he has to go to bed. His grandma told me he doesn't sleep well and it's my job to make sure if he escapes, to get him. It might seem comical but I'm kind of nervous. His grandma and grandpa need to wake up early to go to work, so it's got to be quiet of course. Any ideas on how to make sure my little cousin stays in his bed to go to sleep? I'm a 15 year old girl by the way, if that means anything. The twins are 4, and I think their almost 5. (The other twin should be fine).