here's my thought....start the top as you would for a top-down raglan (you can use any of the formulas on the net,
here is one), figure out the math ahead of time so that you can merge the two patterns successfully when you split for the sleeves. Ignore the increases at the beginning of the rows in the dress pattern (those form the v neck). Once you get past those increases, follow the dress pattern.
The top will be a slightly looser fit this way (I'm guessing). Make sure you do a very loose cast on so it will fit over your little girl's head. The good thing about top down is that you can have her try it on as you go....allows you to tweak things a bit.