Squats, situps and any repetitive exercise that stimulates your cardio vascular system before bed is a good idea because it burns off the excess energy you've got in your system before it turns to fat as your body breaks it down and stores it.
You also won't be supplementing that energy with a new meal until the next morning.
Some people also report that mild exercise before bedtime helps them sleep better too, so definitely a healthy habit to get into!
As for your butt, it does work those muscles - but really the secret to a great looking booty is a combo of toning muscles that support your behind, fat distribution around your butt area, diet, and genetics.
And then some people are just blessed.