Little kids- especially girls it seems, love miniature versions of things, so tiny as a party theme strikes us as a winner. Think tiny birthday bunting with triangles an inch and a half wide, a tiny cake, and an tiny invitation enclosed inside a larger envelope.
Other suggestions include these Tom Thumb Milk Shakes served in 2 oz cordial glasses with a straw. Grilled cheese can be cut into quarters- each quarter being one serving, and served with gherkins. Tiny art and school supplies in little bags can go home as favors.
We feel like if you start looking around in catalogs or even dollar stores, this theme can really take off.