I was in a situation pretty much just like this, i knew my parents would be supportive but it was till terrifying thought to come out to them because of all the horror stories I heard, but one day I was just kinda done being in the closet. I practiced a speech i wanted to tell them which of course went all out the window when I actually did, but I just went down stairs and sat next to them on the couch and told them i had something to tell them. I started out with my speech but then lost it all and ended up just telling them about how i felt about being pansexual, which is what i am, and how it was a part of me and that I'm not gonna change and that it was just something I wanted to let them know, they hugged me and said they loved me and that this didn't change how they thought of me and just made them even prouder to be my parents. It was a huge lift off my shoulders and I suggest you do it when ever your ready, just by bringing it up and explaining that you are who you are, they might have questions so give them a little while to ask them then lit it be a weight off your shoulders, I'm sure it will turn out great, hope my story helped in any way, but just remember that not everyones the same, your coming out will be different and their reactions will most likely be a little different but i'm sure it will be in the same ball house. don't be worried cause if they are excepting as you say they are I'm sure it will all work out great.