Crocodile campaign aims to stop the rot.
Smiley, a life-size mascot, launched the campaign in Wales at Playbox nursery in Cardiff.
Dentist Graham Wilding, founder of the campaign, said, "As a dentist with more than 20 years of experience, I have seen much suffering from tooth decay, especially in young children.
"This is especially distressing when we know that tooth decay is preventable. Stop the rot aims to promote that prevention in pre-school toddlers.
"By encouraging healthy eating and great oral care habits from when teeth begin to appear, we can help to ensure that children enjoy good oral health in the long term."
Official figures show that:
Nearly half of all five-year-olds in Wales have some tooth decay;
48% of Welsh children under five suffer from tooth decay;
52% of Welsh children over eight have tooth decay;
56% of under-fives in South-East Wales have tooth decay;
53% of under-fives in South-West Wales have tooth decay;
44% of under-fives in North Wales have tooth decay.