Generally the answer is yes. The brit milah ceremony is certainly a family affair as well as a religious occasion. Moreover, the boy having his bris ceremony may already have siblings. Having youngsters participate in any Jewish ceremony is both an educational opportunity and a chance to help foster a child’s identification with Judaism.
A problem can sometimes arise, however, when either the attention span of a younger child wanders or a child is too young to understand the import of the events taking place. Crying, shouting, running around in the mist of the ceremony are distracting to observers, parents, and the mohel and detract from the dignity of the proceedings. The best rule of thumb is to have a responsible person designated to take a child out of the room if he or she is causing a disturbance.
The other issue for children and adolescents is whether or not the circumcision itself would be traumatizing for them to observe. That issue is best addressed by the parents of that child who know better than anyone else whether the child is ready to observe the brit milah ceremony and the actual circumcision procedure.