Kids Lunch box - suggestion for fruit smoothie packaging
You could use a plastic syringe (without the needle obviously), which can usually be found at cake stores and such.

Where I come from there's a chocolate shop that sells plastic syringes filled with chocolate for kids. 

On the other hand, the extended plunger may take up too much room. In that case, maybe a test-tube sort of thing with a plastic top to seal it. I've seen them around, but wouldn't know where to find them.