First off, I'm really sorry for your loss.
I think a long as they are in nice clothes that aren't too loud or bright that they'll be fine. For a girl a nice dress, sock or tights and nice shoes. For a boy nice pants, buttondown shirt, nice shoes and maybe a jacket, sweater or vest.
I think as long as they are clean, their clothes are clean and they use their nice manners that's all that matters.