The word "chemistry" calls to mind vile-smelling concoctions that necessitate the use of lab goggles and gloves. However, in reality, all substances are "chemicals," even everyday substances like water and tasty ones like sugar. Hence, coming up with a fun and easy chemistry experiment that's safe enough to do with kids is quite trivial.
With a 40% by weight solution of Elmer's Glue in water and a 50% by weight solution of Borax in water, you can make a fun slime that will delight a pre-teen (or even an adult!). Just mix equal volumes of both solutions in a plastic bag and knead.
The slime is a product of the process of "crosslinking," in which the long molecular chains of the glue are "tied" together by the Borax. Since the chains are tangled together, they have more cohesion than the water/glue mixture would have by itself. Even the water molecules get tangled up in the water/glue matrix, and hence is remains a gel instead of drying out into a hard mass as glue normally would.
Rock candy is a great experiment in solubility! Sugar, like most substances, is more soluble in water at higher temperatures. Hence, by heating the sugar, you can get it to dissolve in a relatively small volume of water. However, once that water cools, the sugar will go back to its room temperature solubility. Over a period of a couple weeks, the sugar crystals will gradually grow into "rock candy".
As anyone who has made rock candy knows, crystallization requires a "nucleation site". In other words, the crystals need a rough surface upon which to form. That's why old-fashioned rock candy always had a stick or a string. If you try to grow sugar crystals without a stick or a string, then they may or may not form. Ultra-clean, very smooth glass will not support the growth of crystals. Labortory chemists sometimes add what's called a "seed crystal" to get around this problem, or sometimes they use a stick, just as candy makers do.
"Syrup" products are often what scientists call "supersaturated", meaning that the water in those products is holding more sugar than it really can at room temperature. However, because there are no nucleation sites, it does not from crystals, but rather remains a syrup.
On the topic of solubility, note that just drinking a nice cup of cocoa can be an impromptu chemistry lesson. When hot, the cocoa and sugar become suspended or dissolved respectively in the water. If you let your drink get cool, there is usually a brown, sugary lump at the bottom. Why? Because to stay in the water, those substances needed that thermal energy! When the cocoa cooled, the molecules no longer had the energy to stay in the water, and hence they fell to the bottom.