(ps - i'd get the bigger one and size down for the 2yo.
it's pretty easy - work out your 2yo's measurements, and look at the pattern.
then each measurement it gives on the pattern, reduce by that amount.
okay, say that you've got 3 pieces on the front of the dress (2 sides and one centre front) - if the difference in measurements are 20 cm all round, assume 10cm for the front pieces.
you then have to divide that 10cm by the number of seams you have.
for 3 pieces, you've got 6 seams, so you would reduce each seam by 10/6 = 1.67cm.
do the same with the back pieces.
bear in mind that all length measurements will be different too - my recommendation would be th ignore the length, and concentrate on the armholes (this only works for children's garments cos their shapes are not so different). measure the armhole for the older size, then put a piece of ribbon/paper to that exact size against her armpit. (around the upper arm)
see how much it allows for movement, then apply that logic to your 2yo by measuring round her armpit with paper or ribbon.
then measure it (say it's 25cm or something) - work out the measurements on each garment, and divide your ribbon by that much - you will see that it's rounder on the front of the armpit than the back.
you can work it all out on paper by tracing the pieces that surround the armhole of the garment to see the shape, then put your 2yo's ribbon on a new piece of paper to that shape.
then apply that to the (already re-drawn) pieces for the 2ro's pattern.
the length, as i say, don't worry about - just make it up with the side seam/armhole (and the same adjustments for the neck line as for the armhole - use the same method), and put it one 2yo and work out the correct hem length form there.