When one makes a decision that severely affects those closest to one, then one should include them to some extent in the decision making if at all possible.
That is to say, the decision is "yours" in the end, but with something this serious, this big, this.. I lack the appropriate word, but this is a substantial portion of your identity, emotional state, and life, regardless of whether or not you choose to tradition.. and so, it seems like something you should attempt to make known to your significant other if you want to be with her for the foreseeable future at all.
Not to tell you what to do, but it just seems like the right and honest thing to do to at least attempt to approach the topic with her to any extent before you make any choices that would affect her life.
The thing with children is, is that when they're young they have no prejudice or preconceived notion of what is or is not "appropriate" in a sense. That is to say, if you tell a young child that "daddy's not a boy on the inside, on the inside daddy's a girl and she needs to come out a bit" with the obvious "but she'll always be your daddy" and all that jazz, then it's more probable that the child will accept that and embrace that than if you'd tell the same to, for instance, a teenager (Full of societal pressures and preconceived notions and so on...)
As such, waiting for the child to grow up "can" contribute to the relationship failing at a later date, especially if there isn't a lot of care taken to teach it about gender identity and sexuality, and the truth that "heterosexual cisgenders" aren't the only right way to be (in fact there isn't a right or a wrong way to be, just different ways, and that's perfectly okay, and anyone who says different is just being mean).
In the end though, the choice to transition or not is entirely yours and yours alone to stand under and no-one can tell you whether or not you "should". You have to choose based on your own reasons.
As long as the choice is "yours" whichever one you make in the end, then you can live with that choice. It's when we let others, be it friends, family, society, philosophy, religion, etcetera, make the choices for us that we feel the most trapped and unhappy in our choices.
*hugs*
Good luck in figuring it out, and good luck with whatever choice you make.
Keep us updated.