The research on grandparent involvement in early childhood is overwhelmingly positive, and there are specific things grandparents can do that parents simply cannot.
Research by Danielsbacka and Tanskanen in Finland found that grandparental investment, time, play, warmth, stories, significantly predicts grandchildren's wellbeing and developmental outcomes. Grandparents offer something distinct from parents: time without the relentless weight of the parenting role, a different vocabulary of stories and games, and a living connection to the past that children find uniquely fascinating.
The intergenerational play relationship also benefits grandparents: research by Arpino and Bordone shows that regular grandchild care is associated with better cognitive function and slower cognitive decline in grandparents. Real play is good for everyone in the room.
Key citation
Danielsbacka, M. & Tanskanen, A.O. (2012). Grandparental childcare in Europe. European Journal of Ageing · Arpino, B. & Bordone, V. (2014). Does grandparenting pay off? European Journal of Ageing
Many parents find that their screen-time approach differs from grandparents' instincts to offer tablets or television as a treat. The evidence-based approach to this is: provide grandparents with positive alternatives (specific Jaques toys that are grandparent-friendly), share the free grandparent's guide, and frame the preference positively: "She loves playing with her wooden fox when she visits, can you set it out with her?"
Free: The Grandparent's Guide to Real Play → Ideal grandparent-gift toys from Jaques →Britain's oldest toy maker. Every toy open-ended, wooden, and built for exactly the kind of play the research recommends.