It's never easy. When a child is diagnosed with cancer, it makes people wonder how this can happen - how can someone so young get a disease so devastating. Rely on your support system - family members, friends, co-workers. For some the support of someone from the religious community or a social worker can often be helpful. The doctors and nurses taking care of the child may be able to get you into a support group.
With lots of help. With a new diagnosis like cancer, you are going to go through a variety of emotions, most of them related to grief. These include denial, anger, bargaining, depression and acceptance. You need help and support in order to successfully navigate this tempest. Your hospital chaplain would likely be a good place to start.
Social worker. The social worker working with your medical team is an excellent source of finding many types of support for you and your child. They can help get you into contact with other families who have been (or are currently in) the same situation as you and your family. The social worker can also recommend good online support groups if you are interested. You are not alone. There are many that can help.