Most importantly explain that acids and bases are very nasty chemicals that can hurt you and should only be handled by an adult. I wouldn't expect a six-year-old to be able to grasp much more that Mr. Yuck. Even the really bright ones.
I remember when I was seven I watched an episode of Knight Rider where Kit fell in a pool of acid and started to dissolve. I asked my mom what acid was and she said it was a chemical that could eat stuff away. That was a good enough explanation for me. I suppose you could say that bases have an awful lot in common with acids, they eat stuff away, except bases are real good with dirt. That's why soap is a base.
Questions to ask yourself: how "bright," or just precocious; how well do you and the kid communicate: and, who's going to sue you for how much if something goes terribly wrong? There are kids who can be trusted with the information and examples (acid in car batteries, bases in drain cleaners), and there are kids who are going to take that information and develop it --- or, orange juice and baking soda --- messy, but nothing awful --- and, pick up battery and drain cleaner info elsewhere --- also bad scenario.
Whatever you start, you are going to have to finish, and 6 yr. olds can really stretch your resources --- the kid has clearly read the warning labels on every household chemical --- you might agree not to rat him/her out to mom/dad, and explain as many of the hazards as you can, and ask to have any interest in "trying something to see what happens" explained to you first, just in case there's something "you forgot" to mention --- care and respect for adverse possibilities --- respect the kid's intelligence, try conning a kid with horror stories, and you'll be on the (insert)kid's(end edit) "ignore list" in a NY minute.
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