Where we are everyone does bantam sized birds. You often have to hatch many chicks to get a few show quality birds. Bantams reach a size where you can see how they will mature much earlier than Standard/Large size fowl. So it costs less in time and feed to get better results. It also seems to be slightly easier to find breeders of bantam size birds, so you can source better quality birds to begin with. Smaller kids can work with smaller birds more easily.
That said....everyone does bantams so there is more competition in those classes!
You may find that your existing flock does not have any "show quality" birds and that she will have to start from scratch. If so, start with the best quality birds you can. If there are not a lot of chicken fanciers in your area, you may have to choose from the good quality breeds available in your area. So it pays to be flexible, at least for your first set of birds.
There are many lovely picture books of chickens, see if your local library has any or can transit any from other libraries to you. Feathersite.com also has many pics of breeds. She may fall in love with the picture of a particular breed. Then the hunt is on to find good quality birds of that breed!
I imagine you are interested in the temperment and other attributes that are not shown in a picture. I found the chart below useful. Attributes of course vary by individual bird, but I think many of the generalities hold true.
http://www.ithaca.edu/staff/jhenderson/chooks/chooks.html
Now my 15yo boy loves standard/large size Speckled Sussex, but it is near impossible to find quality examples of this breed in our area. So we are working with what we have and understand when we do not get blue ribbons at the shows. Each generation looks a little better than the last.
The 12yo boy perfers our overly friendly Delawares, my husband the meaty-looking Buckeyes, my mom likes the elegant looking Ameraucanas.
I like all of them and want more (chicken math).
Good Luck with your search for your "best breed"!
