Ahhh.... This is so hard they are all so pretty. i'm not a fan of the bride looking ones or the ones that go all the way down to the floor, but i'm sure you could get the seamstress to make tailor it to your needs.
I like dress A the best!!! I like dress F second except i would ask the seamstress not to make it floor length. I like the one on page 3 (organza) third. I also like girl2mama's dresses but i do not think they would work well with your train.
I like the idea of having my train made into a communion dress for my dd
I will have to look into that in a few years.
Also, i would measure your son for an approximate size. I bet your dd will not be bigger than him at the same age. If anything she will probably be a little smaller.