What are the side-effects of swallowing a small amount of stain remover?

I can't answer this exactly as you've not told me the kind of stain remover. However, I am not aware of any stain remover that is safe at all. As such, I have two recommendations for you: * Call the emergency number listed on the container. * Call Poison Control at 1-800-222-1222. They're a nationwide no-cost service that handles this question expertly -- all day, every day. Whichever you choose, I'd call one or the other NOW.