Unicameral Bone Cyst is a condition seen in children as a solitary fluid filled cyst at the end of the long bone of a limb. Often it is seen in the upper part of arm or upper part of thigh. In the non-weight bearing bone, like in the arm, if it is an incidental finding you may just keep the child under observations with precautions in the use of the limb. You have not mentioned whether your nephew has a problem in the lower limb or the upper limb. In the lower limb, there is a risk of fracture, often it occurs with a minor injury. If there is a fracture the fracture heals but the bone may remain weak. It may then need grafting (bone taken from some other site in the body like the pelvis) to improve its strength.
If there is no fracture, injection steroids is an option. Unlike the systemic steroids (steroids given by injection in to the muscles or in the veins) the steroid injection here is active only locally. So there are no generalized side effects. However, there is a small risk of increase infection at the injection site because of the steroid. Therefore, the injection is given under strict antiseptic conditions.
Some have even tried injection of bone marrow. If there is an impending fracture, you may need to operate cure it out the area and pack it with bone graft.
It must be clear to you by now that there is no final method of treatment. Individual centres have their own protocols. In your nephew's case, in a lesion of the lower limb, I would consider injection of steroid / bone marrow with protected weight bearing as an option.