Much has changed over recent years in the technology that children use in their daily lives. The impact of these changes on children's time in screen-based activities has been the source of much debate and concern. Yet we know very little about change in children's daily time in screen-based activities, including their time using devices such as smartphones and tablets. Using data from two nationally representative UK Time Use Surveys 2000–01 and 2014–15, this paper presents a detailed study of change in children's (8–18 years) daily time in screen-based activities (TV, videogames, and computers), together with the latest data on their time using computers and mobile devices throughout the day. We find that children's screen-based activities increased by around half an hour between 2000 and 2015, but that this was concentrated among boys who increased their time playing videogames. Dwarfing this, however, was the additional time children spent using computers and mobile devices when engaging in other activities throughout the day, especially for girls. Multivariate analysis of factors associated with children's screen-based activities revealed that gender differences in children's time playing videogames widened significantly over this period, while socio-economic differences in children's screen-based activities remained fixed. This study highlights how children are combining old and new technologies in their daily lives, and points to issues for future developments in the measurement of children's engagement in screen-based activities to aid in assessing the impact of technology on children's well-being.