Stacks of suitcases etc in front of the door?
Alas, and I'm being honest from the point of view of someone with a totally untrustworthy MIL (and a dead, would have been awesome grandma, mother), if I ask a grandparent to watch my kid, I want them to WATCH my kid. I wouldn't have been OK with you sleeping to begin with.
We traveled with my cousin and her family and my aunt, and my cousin wouldn't have been OK with her mom sleeping while watching their 4/5 year old, either.
Sorry, but it's where I'm coming from. Get a kid in an unfamiliar hotel room in an unfamiliar situation, have parents go out...they need someone watching them. My opinion. My feelings.
With children who could be described as cunning, my plans would not involve a cruise.