Apparently if you mix these beads with tonic water, they glow under a black light. These, of course, would be Grindylow eggs.
What a fun idea!
Someone mentioned cornstarch. You could label it quicksand. I don’t think it needs to go down the drain.
Colored sugar or sprinkles - rainbow powder
Vanilla extract - what to name it?
Cocoa powder - Label it bittersweet as it smells chocolately but tastes bitter.
Salt water or coarse salt - phoenix tears
Eyeball sprinkles - eye of newt
Gummy worms, fish, frogs, etc.
Bones candy - lizard bones
Licorice whips - tail of rat
It’s easy to make magic sand, the kind that doesn’t get wet in water. Just moisten clean sand with regular Rain-X and dry it out. This wouldn’t be for a kid who might ingest it, however.
This milk trick is cool:
Elephant toothpaste (or dragon toothpaste for your purposes :)).
This video should give you a few ideas.
^ Get outta here! You faked all those with yer computer! 
Forget junior wizards–I wanna try those!
Do you think he would like to make his own bath bombs? There are different recipes but this video looks pretty good:
Something more simple would be to make his own bath salts, you can just mix up Epsom salts with a little fragrance oil and food color (optional). He could make gifts by putting his mix into a container that he decorates himself.
Not entirely sure how to work this into the ‘potion kit’ idea, but next time ya’ll mix baking soda and acid, put some dish soap and food coloring in the liquid first. Colored bubbles that last for several minutes instead of immediately disappearing.
Hope that link works – came across it just now, free chlorine and pH pool testing strips!
It would be neat if he got boxes from Hogwarts (or some store in Diagon Alley). You could do cool things with the wrapping or the boxes, and kids love getting mail. If he got something every couple of months (have a friend mail it) to add to his Hogwarts Hope Chest.
StG
It’s not a zombie, it was only hibernating! I finally finished up the Potions Box! It came out wonderfully! Photos:
I found an absolutely perfect old wooden jewelry chest at the thrift store with room for ALL the fantastic ideas you gave me, plus the leftovers from the previous incarnations of the potion kits, plus a generous helping of plastic labware for the kid to use (and quit using all our glass tumblers instead). And I actually finished it before Christmas! I guess now I have to figure out how to wrap it, huh?
THANK YOU ALL
Any way to get Jack Skellington to deliver it? In full Xmas regalia?
And if he doesn’t know Mr. Skellington, it might be time to introduce them?
If he hasn’t had the opportunity to play with it, corn starch is dirt cheap and really fun to play with when mixed with water. It’s the stuff moves around like a liquid but acts like a solid when pressure is applied to it. You can squeeze it into a ball in your hand but as soon as you stop squeezing it just melts in your hand.
This spell could be VERY dangerous, for reasons that will become apparent:
When Wizard, Sr. is sitting in his favorite chair, watching the game (Quidditch, of course), Jr. Wizard runs up to him, yells, “SHAZAM!” and drops a peanut into his open deuce-deuce. And runs like all the daemons of Hell are on his heels.
Finally remembered to check that link while not at work and all I see is:
Oh, are you perhaps not a Facebook Straight Doper? Is there somewhere better I should post photos? You can’t post them here yet, can you? I’m a bit out of touch.
You could use imgur.com! Free to use and you can set pictures to be public or private; others can still view private photos, but only when given/by clicking on the direct URL.
I’m on FB but not part of any SD group.
Oh! OK, try this:
THE POTION BOX (via Imgur)
That is absolutely amazing!! Well done, emm! Take a bow!