Using video-ethnographic data from a 'try-on day' in a bus-based mobile preschool, we discuss how children with different levels of experience collaborate with one another and with pedagogues to socialize newcomers into an ongoing community. Analyses show how pedagogues create moments of collective orientation and, besides through verbal instructions, invite newcomers to participate in core activities with older children. The novices engage in the priming event by intent participation, while the older children assume, and are assigned, the role of more experienced participants -- 'old-timers'. In this collective socialization, old-timers, children, and pedagogues strive to create a smooth transition for novices, particularly by stressing the specificity of participating in a preschool that is mobile and located in a bus. The results illustrate how becoming a mobile preschool child entails understanding and mastering competences such as safely riding the bus, eating meals in the bus, and walking in line in diverse spaces.