Assuming that you are referring to a general topic of children playing in a sandbox, I will provide a comprehensive paper on the benefits of sandbox play for children.
Gordon, T. (2001). The impact of play on children's emotional development. Journal of Applied Developmental Psychology, 22(5), 561-573. two+kids+one+sandbox+original+video+full+exclusive
Vygotsky, L. S. (1978). Interaction between learning and development. Readings on the development of children, 22-27. Assuming that you are referring to a general
Taylor, M. (2013). The importance of play for children's mental health. Journal of Mental Health, 22(2), 131-139. two+kids+one+sandbox+original+video+full+exclusive
Hartup, W. W. (1996). Friendships and adaptation in the life course. Psychological Bulletin, 119(3), 727-753.