Find a dress. It should have sleeves, a flared skirt and reach below the knees. Look for drab colors like brown, dusty rose and maroon in small prints or solids.
Find a pinafore, or an over-sized, scoop-necked, sleeveless dress, shorter than the bottom-layer dress. This goes on over the dress. Or, make a pinafore from a pattern. Distress the fabric and add a couple of stains and patches.
Make bloomers from an old pair of white or gray leggings. Cut them off and add elastic.
Finish up with baggy socks or dark tights and high-top boots.
Dress your boy orphan in a long-sleeve, button-down shirt and with a pair of dark pants. Old dress pant work great, but do not use jeans. Shorten them to about mid-calf and add a patches to knees, elbows or seat. Hold up the pants with suspenders.
Wear high-top boots and baggy socks or bare feet.
Use a flat wool cap with a bill to keep your orphan warmer on those cold nights spent begging on the streets. This also adds pathos when held nervously in both hands.