Can grandchildren help dementia?

Not sure . If you mean can the presence of grandchildren help the demented person- yes. Any physical stimulation can be helpful and grandchildren can certainly be stimulating.
Not inherited. Not all dementias are due to to alzheimers disease. Lets start for the begining: dementia (cognitive functiion prtoblem) can be produced by many diseases that are treateable or arrested included brain tumor, subdural hematoma, depression, medications improperly used, B12 deficiency, etc. . If every thing is excluded then alzheimer and frototempoal dementioa are on top. Only about 1% is inherited.