Last year, medical chiefs recommended all under-fives should receive free doses of vitamins A, C and D.
But 59% of parents are not taking up the advice to give vitamin D, a poll found.
The survey, commissioned by the Health Food Manufacturers' Association, found 64% felt their children received enough vitamin D from their diet.
The poll of 10,000 UK adults found 76% of people did not know youngsters are advised to use vitamin D supplements.
There are rising concerns about the number of children with rickets, often caused by lack of vitamin D, which is mainly made by the action of the sun on skin but also comes from foods such as oily fish.
Experts say a key reason for the problem is that since the 1990s, in a bid to reduce the risk of skin cancer, most people in the developed world have reduced their exposure to sunshine.