quality workbook!
i posted 2 potions in one of the worksheets a while ago - one for blowing bubbles through a ring, and one which will foam and bubble crazily.
the one which foams is a chemical reaction (it looks pretty impressive), releasing carbon dioxide gas - the baking soda neutralises the acid in the vinegar, leaving a solution of sodium acetate in water. if they drink it it will no doubt taste pretty boggin and be quite gassy (burps), but it shouldn't harm them...
i've tried to find other recipes that produce cool effects, but the only other one i found (looks bad-ass but) needs ingredients that aren't easy to find in korea (surprise, surprise haha).