We had to look after our son and DIL's barely-walking toddler for a week while they were on holiday and though she loved being with us she pined by the end of the first day and kept asking and looking for her parents. We comforted and distracted her and looking back it strengthened the bonds between us. She's now 16 and though we live at opposite ends of the country, we couldn't be closer. In fact, on later visits as a toddler she would cry at the prospect of leaving us, every time. I found it comforted her to give her something of mine, like a pretty scarf with my perfume on it, as she sat in the car waiting to leave. Children do see such things as talismans, if you present them that way. Maybe your grandchild's parents could do something similar.