What comes to mind is using liquid nitrogen to turn water into ice. If there is a juice everyone can drink, each child could get a frozen juice cube that started as a bit of juice in a paper cup.

How about making Jello? Liquid to solid, for sure.

One of my all-time favorites as a kid is the Cornstarch mixture. Hard when struck, liquidy when barely touched.

Sorry jsdunbar,but handling liquid nitrogen around kids sounds dangerous.Realy dangerous!I would go with Benny's idea.Sounds fun!

Shame about the eggs allergy - there are all kinds of party tricks one can do with them. (Starting with mayonnaise!) And gluten - showing off gluten strands after soaking dough water is also cool.

But anyway, my mind keeps going towards emulsions. You could make a vinaigrette out of mustard / oil / vinegar and show how that can thicken and bind from the lecithin in mustard? ("Oooh, salad..." say all the kids) Not exactly liquid to solid or vice versa, but maybe there's some other application/tie-in there.

What about making popsicles? That involves a true change of state (which cream to butter isn't, I don't think), and you could make them out of fruits and juices without allergy-inducing ingredients. Kids could eat them the next day or later in the day if you made them small enough. You could do ice-cube-tray popsicles with little sticks stuck in them to minimize special equipment.

Or another one that's fun is ice cream in a coffee can -- you could definitely do it with coconut milk. You can use any vegan or dairy ice cream recipe you like, but here is the basic freezing method: http://crafts.kaboose.com/ice-cream-in-a-can-or-kick-the-can.html

This is the only way I'll make ice cream with kids!

Found this old thread by chance.
Anne-Michelle - what did you eventually do?
If the kids didn't have to eat the outcome, you could do four demos of liquid to solid, one each by chill, churning, chemical, heat (for water, cream, gelatin, eggs) to show that not all liquids react the same.