This is a mistake many of us make: we want to get to a destination so fast that we spring the news on children and expect them to adapt as quickly as we need them to. It doesn’t work that way and we need to remember that a family transitions along with you.
Your idea of transition may be unique to you. It may be that you simply have a feminine persona that needs expressing or conversely that you ultimately need to medically transition. In either case, there must be a slow adaptation to something you may have grappled with all your life, but which they have not had the luxury of being exposed to. We cannot rush our children and expect that they will absorb something this significant in a very short amount of time.
Conversely, the idea that they will never accept us is completely erroneous and some trans people refuse to do something because they think they could never put their children through something like that. Nothing could be further from the truth because people are malleable and adaptable beings who, if given a chance, can grow to accept many situations. We just need to give them the adequate time to do it.