Abstract Background: Childhood obesity has increased steadily over recent years and has coincided with a general trend towards larger portions of foods consumed both inside and outside the home. A causal link between portion size and weight gain has not been established, although there is evidence of an association between larger portions and greater energy intake. The present study aimed to investigate parent’s attitudes, knowledge, practices, and concerns about appropriate portions for children. Methods: Four focus groups with a total of 14 volunteer mothers of 8–11-year-old children taking part in a larger school-based study. Mothers were asked their views about portion sizes for their children and were asked to demonstrate typical servings that they would offer their children, by weighing five common foods provided. Conversations were tape-recorded and transcribed for thematic analysis. Results: Participants were unconcerned about portion sizes and would not welcome official guidance, particularly if it involved weighing foods. Mothers fed their children the amount that they believed they would eat and felt that this varied across children and across occasions. The weighing task revealed a wide variation in portion sizes served to children, with portions of the more energy-dense foods being smaller on average than those of less energy-dense foods. Conclusions: There was little understanding of age-appropriate serving sizes amongst mothers in this study. Education campaigns should be mindful of the need to make portion information clear and simple because parents may not be prepared to weigh the food that they serve to their children.