Thus far, daily diary studies have mostly focused on adults and adolescents, while only a few have researched young children. In this methodological article, we introduce and evaluate a mobile diary application, “You have a message from Illi”, designed to capture young, under-school-age children’s daily experiences at home and in day care, in the context of a 24 h economy where parents work nonstandard hours and child care is arranged accordingly. We also compare childrens’ mood ratings reported by both the children themselves and adults. Children recruited either from day care centers (N = 15) or day and night and care centers (N = 17) carried smartphones with them for 1 week and reported their daily moods and activities three times a day by mobile phone as motivated by “Illi”, a fairy-tale character. The questions concerned daily activities, moods and transitions as well as good and challenging moments. The study demonstrated children’s ability and motivation to participate in a mobile diary and produce valuable information about their daily lives. The method captured fluctuations in children’s daily moods, as reported by both children and adults. The mood evaluations of adults and the children themselves correlated. Parents and day care staff perceived participation in the mobile diary study as practical, relevant and not over-demanding, and that the method was easy to integrate into daily life.