Japanese kindergarten children usually bring lunch prepared by mothers. The contents may be influenced by mothers’ food concerns. We investigated the relationship between mothers’ concerns and children’s lunch box contents and preferences. Lunch boxes of 209 children were digitally photographed for 4 days at a private kindergarten in Japan. The amounts of rice, main dishes, vegetables and fruits in the lunch boxes were estimated by measuring the area occupied by each in the photograph; a questionnaire, including questions on mothers’ concerns and children’s preferences, was completed by mothers. Vegetable amounts in the lunch boxes were significantly related to mother’s concerns for their children’s lunch. Compared with estimated vegetable amounts below 11%, the amounts above 11% indicated that the number of foods disliked by children was lower, and mothers reported a higher rate of mindfulness towards vegetables and lower rate towards frozen food and believed that they prepared a balanced lunch. Thus, vegetable amounts in children’s lunch boxes, estimated using photographs, may predict mothers’ food concerns and children’s balanced/unbalanced diets.