How to get help for children with cancer?

Various. Here are some suggestions: hospital social worker, local clergy, teacher at school, local or distant support groups, internet resources such as cancercare.Org.
Go to doctor. If you are talking about medical treatment, then you must go to the doctor for that. Once your child starts getting treatment, he/she will be be able to get help from multiple community resources available through different social services. An office or hospital social worker would be your reference person.