You should be fine to get back to MK before 9pm, even 8:30 is doable. Unless your "parents" aren't able to I suggest simply walking back to MK. Its a pleasant walk that will take 10 mins or less and will help digest that meal. It will also "free" to have that bit of extra dessert at the make your own sundae bar at CM's.

You'll also avoid the possible crowds on the monorail.