Short answer: use Qwerty.
Long answer: it depends:
Why do you want to introduce Dvorak? Hear me out -- I think Dvorak is great but the mere fact that it's not the universal default makes for a difficult reality.
First: The difference lies in typing comfort, not speed. There is no noticeable speed difference between Qwerty and Dvorak, provided that the test subjects are well trained in their chosen layout.
I have no hope at all that Dvorak will slowly become the dominant layout - ever. It's been around nearly as long as the Qwerty layout, and even in this millennium its usage is limited to those experts/geeks who care about this - but they are less than 0.001% of the workforce.
Still, using Dvorak would be recommended if you're J.K. Rowling or Stephen King (or an average office worker) just because you'll be typing a lot. But if you won't be using a keyboard most of your career, then Qwerty is probably the wiser choice simply because it is the universal default.
Using Dvorak is neat. It's comfortable. Learning it is a horrible pain for those who were already proficient in Qwerty before they started with Dvorak. If you're a hunt-and-peck typist then you won't even notice the different layout :-) and if you're just starting with typing then both would be learned equally well. This is of course what you're thinking of.
But realize that for someone who is proficient only in Dvorak will have a hard time when he/she is not at his own keyboard. (I use a Danish layout but live in Austria, that little bit of layout difference is enough of a challenge!) There will be plenty of situations where you'll be facing a Qwerty layout, and you won't have the luxury to simply install Dvorak before you start typing. Many businesses restrict the software permissions, and many times in life you'll be borrowing someone else's workplace for a limited time. Also: shared home computer?
There are countless situations where this is going to be bothersome, and you'll feel like an outsider, and you'll be dealing with questions about why you're weird. This is reality, unless you practically never use anything but your own computer.
Do the benefits in comfort outweigh the trouble it brings? That's an individual assessment, and you can't know what your child will do in the future. Your idea is commendable, but I think there are bigger battles to fight, more important issues in life to spend your energy on.
Source: I've been proficient on Qwerty for 25+ years and started using Dvorak a few years ago - but still haven't made the switch full-time, in part because of workplace difficulties.