We did a solubility station that both my then-3-and-6yo kids loved. The trick is to require that they hypothesize about what will dissolve and what won't before they test it.

Testing Ph is also fun with kids that age. You can make your own (stinky) Ph indicator with red cabbage. Just don't boil it the day of the party...or even the day before. Pee-yew!

Sounds like a fun party! smile

Kriston