The Young Person's Food Atlas (YPFA) is a book of photographs of food portion sizes developed for use in assessing the dietary intake of children aged 18 months to 16 years in the UK. There are three versions of the atlas available, for children of pre-school (18 months to 4 years), primary school (4 years to 11 years) and secondary school age (11 years to 16 years). Parents and their children were recruited to take part in a relative validation of estimates made using the YPFA against four-day weighed food diaries. The parents kept a weighed diary of all the foods their child consumed for four days and observers weighed and recorded foods consumed at school or nursery. Interviews to gain estimates of portion size were conducted on day 5 with parents and, where appropriate, with the child themselves. These data comprise the individual estimates of portion size made for each food. They include information on the weight, as estimated using the YPFA and the weight recorded in the food diary/weighed by the observer, along with information on the child's age, gender, Index of Multiple Deprivation (IMD) and whether the estimate was made by the parent or the child themselves. A separate CSV file contains data on the mean daily intake of energy and nutrients as reported in the weighed food diary and using the YPFA.