This is the model we are loosely basing your pallet play structure on. The latter images are ideas for the structure you had mentioned in our meeting of possible ad-on activities for the children, like:

...a table with a cutout hole for metal bowls to play in.

Small shelving can be added in the structure for toys and tools and so on.

This is a rather "large" example, but I can build a smaller version of this and use a part of the structure wall for hooks. And it would be fun to have the small table with multiple bowls of different sizes the children can play in.

We both liked the idea of having a space "under" a structure on stilts, but this is a cool idea to have a small lofted space for the children to climb into...like a lookout tower.  This structure and how it's positioned to the brick wall is similar to where we're putting the structure next to the fence. Building a narrow step-ramp up to the lofted space would be cool! Just an idea!

This is just an added inspiration I thought was cool. But we can figure out these details after the structure is built.  I just think it's a cool way to store little items in tandem with it functioning as a little desk.

Little Green Schoolhouse

These are just a few examples of "pallet" playhouses.

This is a type of roofing material I like to work with. Whether galvanized steel or plastic, is easy to install and an excellent material for northwest weather. A gutter system can be attached to the structure as well if needed. 

Archaeology Learning Box

Kids (and adults!) like to dig!
An Archaeology learning box can be a creative activity for children to learn about rocks, bones, and anything you choose to bury in the dirt. With proper tools and instructions, children can delicately expose little treasures.
For example: Maybe one lesson you can bury an assortment of rocks like agates, crystals, basalt rock, red jasper...etc. The teacher can provide a list of what is buried in the box. With every stone that's discovered, the children can try to match each stone to the list provided and learn about what each stone is made out of.
Each lesson can be simple, but fun!
You can bury small letters that the children can discover and then match the found letters to a list of words they are learning.
You can get inexpensive little dinosaur toys and they can learn about each dinosaur they find.
The creative options are endless and can be a great way for children to properly use small tools, brushes, little sifting trays and it's a great activity for children to work together as they discover the mystery lesson during each dig.

I can build a small box, sifting trays and provide fun tools.