Dresses for children should always have two shoulders and at the very least they should be hemmed around the armpit. Little girls should not be dressed in dresses that have no shoulders, halters, or dresses that have only one shoulder covered. These are babies and they should be allowed to dress like babies until the time they are grown. If you allow them to dress too grown up when they are small what message are you sending them and the world?
Little girls are not little women. They are girls and they should be dresses appropriately for their age and for the activities they will be engaged in. When a child will be playing outside they should have on shorts or jeans instead of dresses. At the very least they should wear bloomers under the dress that will cover their underwear.