Usually the pediatric oncologist and the oncology team which includes nurse practitioners and medical social workers will share this information in an age appropriate manner with the child.
Children should be told the truth about their condition unless they ask not to be told. Kids are smart . . you can't really hide this type of information from them in today's world . . they'll figure it out . . so it is best if the medical team tells them with the parents there too. This way the child can ask as many questions as he needs to . . and get real answers. It would be impossible to keep this from him anyway, especially if he undergoes chemotherapy, radiation, and surgery . . he would want to know what is going on.
Basically, when my son was told he had cancer . . they just gave it to him in total facts . . exactly what it was . . and where it was . . and what treatment was available. The doctors also told him it was serious, but to ignore statistics. Even though the information they delivered was horrifying and hard to hear . . it was necessary so that we would know what it was and how hard to fight it.