This paper presents an observational study conducted at the public environment investigating children's engagement with a social robot within a pretend play scenario. A pretend play system was designed in order to address the challenges of evaluating child-robot interaction by exploiting the advantages of ubiquitous robotics and ambient intelligence of multimodal, multi-sensory perception. The system performed successfully at a children's play centre where a humanoid NAO robot was able to dynamically adapt its gender by changing its gendered voice to match the gender of the child. By analyzing the free play of children, the results obtained confirm the hypothesis of gender segregation within child-robot interaction. These findings are important and ought to be considered when designing robotic applications for children in order to increase robot's perceived likeability, acceptance and engagement.