Depends on age. Children over 12 are usually given the achenbach youth self report (ysr), to match with parent & teacher reports. The ysr is a statistically proven tool in developing a diagnosis of childhood behavioral or learning problems. See http://www.Columbia.Edu/cu/ssw/projects/pmap/docs/nohysr.Pdf.
Depends. It depends on the presentation, child's age, level of maturity, cooperation, and physical /mental ability.There are several questionnaires such as the ysr, conners sr, basc2 that have rating scales for children 12 and up.Also it is imperative that as part of the diagnostic interview that the child is included.Not including the child in the diagnostic process could lead to mistrust and poor compliance.
Generally No. Children generally are not asked to fill out these assessment forms, but people around the children are. Thus parents and teachers often fill out these forms, which are very important in helping the diagnosis.