Trans Youth Equality Foundation: The Trans Youth Equality Foundation is a national 501(c)(3) non profit foundation that advocates for transgender, gender non-conforming, and intersex youth ages 2–18. Their Resources for Parents Page features an excellent book suggestion list, a guide to supporting transgender students in school K-12, and many other resources.
Movement Advancement Project: Resources for Families of Transgender and Gender Diverse Children, including: resources for understanding who transgender youth are and what challenges they face, why family acceptance is so important, and practical ways for families to support transgender youth.
Gender Spectrum has an excellent guide to understanding gender, including the many dimensions of gender such as body, identity, social gender, congruence, personal gender, and the difference between gender and sexual orientation.
Are You An Askable Parent? is an easy-to-read resource that can help guide you to being a parent whom your child sees as approachable and open to questions.