Simple clothing with solid colors are the best way to show off a child's portrait. You don't want to overpower the child and take away from their face or expression with too bright a color or busy patterns. Another good rule is that if the child is over 2, covering most of their arms and legs brings back the focus to the expression on the child's face. I do love bare feet! The dress or pants that the child wears should be over the knee if not longer to cover properly when they sit down so the camera doesn't look up their dress or see their underwear. Don't worry too much about shoes or belts as these rarely match the clothing and become a distraction. If their mother, father and other children are in the portrait together, the older children and adult's feet will normally not even show as they will be often cropped off just below the knees to bring the portrait closer in to show the smallest child's face. All of the clothing in the portrait should complement each other. A good rule is to go with all formal or all casual, not mix them. Have everyone wear all dark tones or all light tones, or even all-medium darkness. Colors should follow the same rule and be complementary to each other and to the place the portrait might be displayed. If you intend to hang a wall portrait over the fireplace and the room is all in blues, you would want to consider clothing colors that would go with it the same way you would pick pillows to complement a sofa.
A simple classical study of a beautiful child is timeless. This has always been one of my favorites.