A survey has found more than one in five under-fives were being left to brush their teeth unsupervised.
A quarter of parents wrongly thought children did not need to brush twice a day, and 67% thought brushing for one minute was enough – two is recommended.
The survey, of 1,000 people, was carried out by the British Dental Health Foundation (BDHF).
The results also showed that 23% of those surveyed thought there was no need for children to avoid fizzy drinks, even though they have been linked to dental damage.
Dr Nigel Carter, BDHF chief executive, said: “These results really are very worrying and help explain why around half of children under the age of five currently have tooth decay here in the UK.
“Not only has research shown that people who learn good habits as children are far more likely to carry them into adulthood, but taking bad habits into adulthood will cause gum disease and this has been linked to all manner of serious conditions including diabetes, strokes, heart disease and low birth weight babies.”
Janet Clarke, of the British Dental Association, said teaching children how to brush their teeth was a vital part of oral hygiene.
She said: “Twice-daily brushing with fluoride paste should begin as soon as the baby teeth begin to erupt and will need to be performed or supervised by an adult until children are around six years of age.”