Insurance companies generally require that any person living in the household be listed on your policy regardless of driving status. If a member of your household won't be driving your vehicle then you can normally exclude them from coverage or have them listed as a non-driver.
Assuming that your son and daughter-in-law maintain a homeowners insurance policy of their own, it would not be necessary for you to add them to your policy. Their personal property and liability will still be covered by their home insurance provider while they are staying with your temporarily.
A golf cart is covered on a Special Lines insurance policy since it is a different type of risk from a traditional automobile. Unfortunately that means you won't be able to add it with your vehicle, but most insurance companies offer discounted rates for "bundling" your two policies.