A lot depends on the age and size of your junior bridesmaid. If she is tall enough, you might be able to find a small enough dress within the regular bridesmaid dress selection. Sometimes it only takes a bit of custom fitting to make the dress match the rest of the wedding party.
Younger junior bridesmaids present a bit more of a challenge. While some retailers of bridesmaid dresses may carry separate lines of dresses for young bridesmaids, not all designs have options for small girls.
One option to consider is having a different design of dress for the junior bridesmaids. This is particularly useful if you have more than one. The design can be complimentary to the rest of your wedding party, without forcing you to limit yourself to the same design as your older bridesmaids.
Another idea is to have your bridesmaid dresses custom made. A good seamstress can alter a design to fit a variety of shapes and sizes without losing the uniform look of your wedding party. With the wide range of designs and fabrics available, you can come up with a dress that will not only fit all your bridesmaids, but will match your color pallet better than the average dress off the rack.
Sometimes, if you look beyond the traditional bridesmaid dress options, you can find a wider range of sizes. Try looking at prom dresses, or other generalized dresses at department stores. Often these dresses are available in a wider range of sizes, both for adults and for children.
Junior bridesmaids are a lovely addition to a wedding party, and can really add to your ceremony. Whether you choose to outfit them in dresses that match the rest of your party, or choose a slightly different design that compliments your wedding, they will be glad to be a part of your special day. Don't let the difficulties in finding the right dress stop you from letting them be a part of your ceremony.