Many parents provide their newborn son with a fancy outfit or family heirloom for their brit milah ceremony. Yet other than white being a traditional color chosen for the baby’s garment, there is no rule about what a baby should wear at his bris. For practical reasons the outfit should be one that allows the mohel easy access to the baby’s genital area and that provides good visualization for the performance of the circumcision.
Some parents attempt to tie a yarmulke with strings onto the baby’s head. These rarely stay on—and babies do not seem to like them–but there is probably no harm in trying.
The one thing that is important is that, to the extent possible, the baby’s diaper be clean at the beginning of the ceremony. Although infection is extremely rare even if stool gets onto the circumcised end of the penis, it is always better–and certainly more aesthetic–for the mohel to be working in a relatively clean field. That having been said, babies have a mind– and bowel habits– of their own.