Great news about the eggs hatching. We are assuming that Mom and Dad are with the young swans. If so, the adult birds will teach the young what they can and cannot eat. Boxwood should be fine, but we would hold off putting them in for at least 2-3 weeks. This will allow the normal vegetation (aquatic and other) to come back from the winter so that the plants you put out aren't readily eaten to replace normal flora.

Once the swans are acclimated to the vegetation in and around the pond by their parents, then you can introduce the shrub. You also have to be careful not to use any pine straw or pine mulch because this can disrupt the oil in the young bird's feathers. Also, you don't want to dig up any fresh dirt at this time so that the young cygnets get into fungal/mold areas which can make them sick. Again, 2-3 weeks, should be just fine. The Regal Swan