I'd assume that for the purpose of the cruise,
DCL would consider Grandma the guardian since the child is booked in Grandma's cabin, so they're the only adult that Disney can connect to the child. But what do I know?!

Last year we sailed with my nephew without his parents along. He was booked in my Mom's (his Grandma's) cabin. His Grandma who was booked into his cabin had to sign the specific form in question allowing one of us (his Aunt and Uncle) to leave the ship with him without her (which we did end up doing because Grandma was unexpectedly sick and didn't leave with us at one port). Note that in this case we also had another form signed by his parents allowing Grandma to take him on the cruise. The OP's case would be different because the minor booked in Grandma's cabin would have the parents on-board.