One-year-old Daniel Williams took hold of the small black bag from Kate, wife of heir to the throne William, and started playing with it as she chatted to his mother Lucy.
Lucy, who was holding her son, told him to give the bag back and tried to wrestle it from him before Kate indicated he could keep hold of it while she greeted other members of the public.
"Don't eat it now," Lucy told her son while Kate's back was turned.
The encounter occurred on a visit by Kate and William, the Prince and Princess of Wales, to the village of Aberfan to commemorate a disaster where a coal-tip landslide destroyed a primary school in 1966, killing 144 people.
A Kensington Palace spokesperson said the couple "recognise the huge significance of this historic disaster to the people of Wales and wanted to pay their respects to those whose lives were lost as well as to meet people living in Aberfan now."
(Reporting by Alistair Smout and Michael Holden, Editing by Kylie MacLellan)
