This sounds like an obvious thing to consider when choosing your outfit to go to school or work, let alone to have a professional family portrait taken. Often what happens is clothing has stains that you may not notice in everyday wear. This includes light colors and dark colors. Take a good look at your outfits, stand back while they are hanging on the hanger and check it out in good light. For small children, consider bringing clothes for them to change into once you get here. And bring a burp rag of course for baby drool.