Sometimes age brings out stains deep in the fibers that weren't noticed before. These stained items are great for things to wear when doing a messy activity. It's not snotty at all to want a child in clean clothes when going out and about. The only snotty thing would be to have an unwillingness to put a child in something stained when finger-painting or something. You can try washing and sun-bleaching these items otherwise, or even dying them another color.
Rather than trash, always either donate to a charity (they can always find a use for them, even if they end up being used as rags), or offer them for free as playclothes on CraigsList or Freecycle (look on freecycle.org for the group nearest you, and people who want it will pick it up from you).