Well its up to you what you say or show since you know them best...
the NHS brochure explicitly states for trans people, their families and healthcare staff...
and the unicorn dads website is specifically for parents...
one strategy could be to explain your feelings, show some materials and tell them you want to see a good gender therapist...
the gender therapist to help you along, maybe with easy reversible steps first, to help you find out how they make you feel...
so that you have someone to guide you and help you along, and no rash but appropriate steps are made.... appropriate also in your favour, so that it moves along...
oh and here is another resource that might help...
hawaii.edu/hivandaids/Some_Considerations_in_Coming_Out_Trans_to_Your_Parents_and_Family.pdf
hugs