Sounds great. I would however re-thing the idea of using balloons. Small children have been known to choke on them if they break. We weren't allowed to use them when we had events at a children's bookstore I worked at for that reason. Maybe some other material for stress balls.

One thing that I've seen used in "therapy" situations for calming is tactile therapy. Get a few squares (say, 6"x6" or so) of different textured fabrics. Some good ones are satins, polar fleece, flanal, denim, and so on. Sew them together like a book. Different children prefer different textures, and the book allows them to find a texture that they like and is small enough to hold in their hands. For sanitary needs, it's best to find materials that can be machine washed.

As for stress balls, rather than the balloons, you could try making foam blocks or balls - like baby toys - that they can squish or toss safely. Balloons may pose a problem because of latex allergies (I know this from personal experience).

A more "re-useable" option for the bubble wrap would be a clicker like those used in dog training.

Dice and a dice cup can also be very calming - shaking and tossing create different sounds and the repetitive motion is, again, soothing.

Rain-maker style toys are also an option - I know there are directions on the web for making them.

I used to work with dual diagnosis (emotionally and cognititvely disabled) adolescent boys, and employed some of these tactics. I hope my input is helpful to you.