You mention that the grandmother has a child-like mentality and lives in some type of care facility? It sounds as though she might possibly be in dementia?

If that's the case, and you have not done outings like this with her regularly (and especially recently), it may be ill-advised to do that to her. People with dementia, even those who seem more easy-going, can get very upset and agitated when they're out of their comfort zone. An overnight trip to a strange place has the potential for very real and scary problems. Have you talked to her day to day caregivers? Her doctor? They may advise you against the scenario you've described.