Organizations:

Gender Spectrum

Trans Youth Equality Foundation

TransYouth Family Allies

Parents for Transgender Equality Council (HRC)
(PTEC Launch Video)

Family Acceptance Project

PFLAG

Books:

Transgender Friendly Young Children's Books

Transgender Affirming Books, Including for Teens
 

Research:

Mental Health of Transgender Children Who Are Supported in Their Identities

Parental Support and Mental Health Among Transgender Adolescents

Gender Cognition in Transgender Children