Grandson visit grandma and granpa with his friends. Grandma tease them a lot. Starts wearing revealing clothes later transparent ( without bra and pantie )
Friends start using grandma for theyr pleasure in many ways ( bdsm stuff etc. ) Grandpa will hold grandson so he wont peek at them what they do but moans will be heard by him clearly. Grandpa will get a boner and each time will be more bold to touch himself in front of grandson while grandson might be curious about his boner. Grandson will tell grandpa that he's feeling weird about those sounds and grandpa will answer him to call his mom if he want to go to home. He will call her tell her about sounds grandma do etc and after a few minutes grandpa will take the phone and go with the phone to the room where friends of grandson are making out with grandma.
Later mom comes to them with daughter. Son will think that she comes for him but she will stay for a while. Grandpa will now use them and have fun as well.
Now nothing holds grandson from peeking. Will he ?
I think ''cuckold'' will suit him best. Maybe older womans will teach him how to be good boy and will be punished for it. strap-on on him while daughter may be used by grandpa.