From the usual ethical violation of human research methodology; as common sense, what if the child's saliva is contaminated with a dangerous life-threatening Herpes virus? Would you give it to any of his/her grandparents?
I think its totally inappropriate, Hazardous and may negatively affect the health of the elderly.
Saliva is considered a liquid biopsy, so imagine taking tissues from some one and put it into another without any kind of investigations and specially with no need.
Beside children especially are exposed to may types of bacteria and viruses that a geriatric patient may not be able to handle depending on his age and medical condition.
Oral transmission refers to the spreading of microbes through saliva or shared foods and drinks. When a person accidentally consumes microbe-contaminated items, such as saliva during kissing, the swallowing action of the tongue wipes the microbes against the back of the throat, allowing the microbe to enter the body.