Many factors affect the artistic success of any portrait, but in general, the simpler the clothing and props, the better the photograph.
Whites and pastels work well if you would like a light background or set. At this age, a baptismal gown looks great, or perhaps, just a diaper cover and their favorite blanket! The process can be wet and messy too so be sure to bring plenty of wipes and burp cloths!
Depending on the style you prefer, the portrait can be either traditional and classic, or playful and casual. A child's personality can shine in understated outfits that express their personality and reveal favorite playtime activities. For a classic look, deeper jewel-toned colors look splendid. For something more casual, denim is a wonderful choice. Favorite toys, books or objects from home give it a personalized touch.
Clothing, locations, and props that show moods, activities and achievements are good choices for these ages. At these ages, black and white, on location, or outdoor portraiture are styles you may want to consider. Again, clothing should be simple and fit the style you are looking for, either classic or casual.
When more than one person is in the portrait, clothing must be coordinated so that it stays within the same style and tonal range (dark, medium or light). A suggestion is to look at which room in your house the portrait would be placed. Is it a casual room, like the family room, or a formal living room? Pick an accent color or two from the room, dress your children in these colors.