Do not forcibly retract the foreskin
During pregnancy, the foetal foreskin appears at week 8 and covers the head of the penis by week 16. From this point on, the inner lining of the foreskin is physically joined to the lining of the head of the penis.  In other words, the foreskin is fused to the head of the penis.
Due to the foreskin being fused to the head of the penis, the foreskins of young boys are not retractable. Forcibly retracting the foreskin can result in pain, bleeding, scarring, infection, or the formation of abnormal adhesions that may require surgical correction. Do not retract intact children. Only clean what is seen.
Over time, the cells joining the inner foreskin and the head of the penis naturally ‘dissolve’ so that the foreskin becomes retractable. It is normal for the foreskin to not be retractable until puberty and in some cases even early adulthood.  Do not retract the foreskin until it has naturally detached itself from the head of the penis.
Do not use soap to wash behind the foreskin
The inner lining of the foreskin and the head of the penis are mucous membranes. Soap dries out these mucous membranes and can result in irritation, inflammation and increased risk of infection. Even when the foreskin is retractable in older boys and men, they should never use soap to wash behind their foreskin; retracting and cleaning with clean water is all that is needed.
 Camille, C.J., Kuo, R.L. & Wiener, J.S. (2002). Caring for the uncircumcised penis: What parents (and you) need to know. Contemporary Pediatrics, 19(11), 61.
 Kayaba, H., Tamura, H., Kitajima, S., Fujiwara, Y. & Kato, T. (1996). Analysis of the shape and retractability of the prepuce in 603 Japanese boys. Journal of Urology, 156,1813-1815; Oster, J. (1968). Further fate of the foreskin. Archives of Disease in Childhood, 43, 400-403; Gairdner, D. (1949). The fate of the foreskin. The British Medical Journal, 2(4642), 1433-1437.