Anyone with knowledge of youth justice?

I need some advice here - a close relative under the age of 18 has a mental health problem (for which bugger all help has been forthcoming over many years) and has needed to be arrested for actions that are a manifestation of this illness. Where can parents get advice about the child's rights and their rights to be consulted etc?

I know that there are some Gransnetters with knowledge in this area and would be grateful for any advice or helplines who might be able to assist in this rather urgent situation.