They also allow them to laze about inside, playing video games or watching TV, instead of running around outside, it is alleged.
And they reportedly tend to smoke in front of the kids, exposing their young lungs to second-hand tobacco smoke and setting a bad example.
Scientists found that the risks were “unintentional”.
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.
Lead author Dr Stephanie Chambers, of the University of Glasgow’s Public Health Sciences Unit in Scotland, said:
“While the results of this review are clear that behaviour such as exposure to smoking and regularly treating children increases cancer risks as children grow into adulthood, it is also clear from the evidence that these risks are unintentional.
“Currently grandparents are not the focus of public health messaging targeted at parents and in light of the evidence from this study, perhaps this is something that needs to change given the prominent role grandparents play in the lives of children.”
The Glasgow team analysed data from 56 studies from 18 countries that included information about the influence of grandparents on their grandchildren.
Overall, grandparents were found to have an adverse effect – despite meaning well.
In many cases, such as rewarding good behaviour with sweets, they were putting the health of their grandchildren at risk with kindness.