The poll of 1,000 parents also found that 79% of children aged one to four are often given more than the recommended portion size for their age.
The Infant and Toddler Forum (ITF), which commissioned the research, is warning that parents are increasing the chances their children will become obese.
As part of the study, parents looked at pictures of food to select the portion sizes they give their children, and were asked how often they give children certain foods.
The photos included snacks such as oatcakes and cheese, and popular meals such as spaghetti bolognese.
The results showed that around one in 10 parents usually serve their child close to an adult-size portion of spaghetti bolognese or cheese sandwiches.
Overall, 10% of all respondents gave snack portions that were too high. When it came to looking at pictures of cheese and oatcakes, 27% selected a portion size that was too big.
Some 71% of parents also routinely offered their child a bigger portion of crisps than recommended.