I assume your child was born a male and is transitioning to female, so I will refer to your child with the proper pronouns: her and she. I would encourage you to do the same because it communicates support and acceptance.
It certainly sounds like this is a big change for you and for your family. I am glad that you will be supportive and are not trying to make this about yourself, because this is about your daughter first and foremost. She is finally going to be able to be on the outside who she really is inside, and that is a beautiful thing. I would try to focus more on the positives of this rather than the potential negatives.