The extraordinary claims are based on a review of research into the influence grandparents have on lifestyle factors that can sow the seeds of cancer in later life.
Grandparents are overfeeding children and letting them get away with not exercising when they are in charge, an overview of 56 studies in 18 countries has concluded. Grandparents overall were found to have an adverse effect on the health of the children despite meaning well. "There were also negative impacts as a result of tobacco smoke by not complying with parents" wishes regarding second-hand smoke and role-modelling negative behaviour.
Grandparents pose a potential health risk to children and may even put them at risk of cancer, scientists say.
None of the reviewed studies took into account the positive emotional benefit of children spending time with their grandparents, the authors said.
Poor diet, excess weight, smoking and lack of physical activity are all known to increase the risk of cancer, said the researchers.
Changes in social conditions, such as more women in the workforce, childcare costs, and a rise in lone parenting, have led to a rise in grandparents looking after their grandchildren.
The majority of studies included in this review looked at these issues from a parent's point of view, and also found that the types of grandparent behaviour described by the parents also caused tension in families.
Lead author Dr Stephanie Chambers said. "Given that many parents now rely on grandparents for care, the mixed messages about health that children might be getting is perhaps an important discussion that needs to be had".
Similarly, 25 studies suggested that children...
"With both smoking and obesity being the two biggest preventable causes of cancer in the United Kingdom, it's important for the whole family to work together".
Candies were also often used to reward, express love or strengthen the bond between grandparent and child.