Probably my favorite kitchen experiment has always been to put water and pepper into a small bowl, and then dip a toothpick with dish soap into the middle, causing all the pepper to instantly and dramatically scatter away from the toothpick.
YMMV of course, but I always felt that it demonstrates something that's not all that visible and obvious otherwise - I feel like a lot of kid-friendly science experiments give an expected or intuitive result, whereas soap's weakening of intermolecular forces in such a dramatic and sudden fashion manages to connect the micro world with the macro world in a clearly visible, almost shocking way, while neatly demonstrating both why water is special and why soap is helpful.
YMMV of course, but I always felt that it demonstrates something that's not all that visible and obvious otherwise - I feel like a lot of kid-friendly science experiments give an expected or intuitive result, whereas soap's weakening of intermolecular forces in such a dramatic and sudden fashion manages to connect the micro world with the macro world in a clearly visible, almost shocking way, while neatly demonstrating both why water is special and why soap is helpful.
I dunno, I just always loved it.